• Platelets (thrombocytes) are cytoplasmic fragments of megakaryocytes, large bone marrow cells.

• Megakaryocytes extend proplatelet projections into bone marrow sinusoids, which break off as platelets.

• Aspirin irreversibly acetylates cyclooxygenase-1 (COX-1) in platelets.

• This inhibits the production of thromboxane A₂ (TXA₂), a potent promoter of platelet aggregation and vasoconstriction.

• Since platelets cannot synthesize new enzymes, this effect lasts for the platelet’s lifespan (7–10 days).

• The other options are incorrect:

  • a) Aspirin does not directly impair von Willebrand factor.

  • c) Aspirin does not decrease arachidonic acid levels.

  • d) Aspirin does not inactivate ADP or phospholipase A₂.

A Howell-Jolly body is a small, round, cytoplasmic inclusion found in red blood cells. It is composed of a fragment of nuclear material (DNA) that was left behind during the normal process of erythrocyte maturation in the bone marrow.

Why the other options are incorrect:

• a) RNA: Cytoplasmic RNA is seen as basophilic stippling, not Howell-Jolly bodies.

• b) Iron: Iron aggregates are seen as Pappenheimer bodies.

• c) Denatured Hemoglobin: Denatured hemoglobin precipitates are seen as Heinz bodies.

• Lymphocytes include B cells and T cells.

  • B cells can differentiate into plasma cells that produce antibodies.

  • Cytotoxic T cells can carry out direct cytolysis of infected or cancerous cells.

• Granulocytes (neutrophils, eosinophils, basophils) are involved in innate immunity and inflammation but do not produce specific antibodies.

• Thrombocytes (platelets) are involved in clotting, not antibody production or cytolysis.

• Erythrocytes (red blood cells) transport gases and have no role in immune responses like antibody production.

• IgE binds to Fc receptors on basophils and mast cells.

• Upon exposure to an allergen, IgE triggers degranulation of these cells, releasing histamine and other mediators → causing allergic reactions.

Why the other options are incorrect:

• a) IgA: IgA is primarily found in mucous membranes, saliva, tears, and breast milk, and its main role is to provide localized protection at body surfaces against pathogens. It is not involved in typical allergic reactions.

• c) IgG: IgG is the most abundant antibody in the blood and is crucial for long-term immunity and fighting infections. It is involved in other types of hypersensitivity reactions (Type II and III), but not the immediate (Type I) allergy mediated by basophils and mast cells.

• d) IgM: IgM is the first antibody produced in response to an initial infection. It is very effective at activating the complement system but is not involved in allergic reactions.

In normal, healthy adults, Hemoglobin A (HbA), which has the structure α₂β₂, is the predominant type, making up approximately 95-98% of the total hemoglobin.

Why the other options are incorrect:

• b) HbA₂ (α₂δ₂): This is a minor adult hemoglobin. It is present in small, normal amounts, typically around 2-3.5% of the total. An elevated level of HbA₂ is a key diagnostic marker for beta-thalassemia trait.

• c) HbF (α₂γ₂): This is fetal hemoglobin. It is the predominant hemoglobin during fetal development and in newborns because it has a higher affinity for oxygen, allowing efficient oxygen transfer from the mother’s bloodstream. Its production declines rapidly after birth, and in adults, it normally constitutes less than 1% of the total hemoglobin.

• d) HbS: This is sickle hemoglobin. It is an abnormal variant caused by a specific mutation in the beta-globin gene. It is not present in normal adults; its presence indicates sickle cell trait or sickle cell disease.

The normal reference range for platelet count (also called thrombocytes) in adults is typically 150,000 to 450,000 platelets per microliter (μL) of blood, which is equivalent to 150–450 × 10⁹/L.

Why the other options are incorrect:

• a) 50–100 × 10⁹/L: This range is classified as moderate thrombocytopenia (a low platelet count). At this level, there is a significant risk of bleeding, especially after injury.

• b) 100–150 × 10⁹/L: This range is considered mild thrombocytopenia. While the bleeding risk is lower than in more severe cases, it is still below the normal threshold.

• d) 450–600 × 10⁹/L: This range is classified as thrombocytosis (a high platelet count). This can be a reactive condition (due to infection, inflammation, or iron deficiency) or a sign of a myeloproliferative neoplasm, and it can increase the risk of blood clots.

• Phagocytosis is the primary function of neutrophils. It is the process by which they engulf and internalize pathogens, primarily bacteria and fungi.

• Once the microbe is inside a phagosome, the neutrophil fuses its granules (lysosomes) with the phagosome, a process called degranulation. This releases antimicrobial substances that kill and digest the bacteria.

Here is why the other options are incorrect:

• a) Defend against parasites. This is the primary role of eosinophils, not neutrophils.

• b) Mediate sensitivity reactions. This is the role of basophils and mast cells in allergic responses.

• d) Neutralize products from mast cells. While neutrophils can be recruited to sites of inflammation involving mast cells, this is not the defining or primary description of phagocytosis.

The metamyelocyte and the subsequent band and segmented neutrophil stages make up the vast majority of granulocytes in the marrow, as the marrow acts as a large storage pool for these nearly mature cells, ready to be released into the blood.

Why the other options are incorrect:

• a) Basophil: Basophils are the rarest granulocyte in both the bone marrow and peripheral blood.

• b) Myeloblast: Myeloblasts are the earliest, undifferentiated precursors and make up only a very small percentage (typically <5%) of bone marrow cells.

• c) Eosinophil: While present, eosinophils are much less common than the neutrophilic series (metamyelocytes, bands, and segs).

• In an infant, the thymus is the primary gland responsible for the maturation and differentiation of T lymphocytes (T cells).

• T-cell precursors from the bone marrow migrate to the thymus, where they undergo selection and maturation before becoming functional T cells.

• While lymphocytes are initially produced in the bone marrow, the thymus is essential for generating a functional T-cell population in early life.

• The other glands listed have different functions:

  • Adrenal – produces hormones like cortisol and adrenaline.

  • Pituitary – master endocrine gland regulating other glands.

  • Thyroid – regulates metabolism, not lymphocyte production.

The megakaryocyte is by far the largest normal hematopoietic cell found in the bone marrow, typically ranging from 50 to 100 micrometers in diameter. Its large size is due to its unique development process (endomitosis), where it undergoes multiple rounds of DNA replication without cell division, resulting in a massive, polyploid cell.

Why the other options are incorrect:

• a) Histiocyte: This is a tissue macrophage. While it is a large cell, it is generally smaller than a mature megakaryocyte.

• c) Osteoblast: This is a bone-forming cell. It is a large, prominent cell, but again, it is not typically as large as a megakaryocyte.

• d) Mast cell: Mast cells are found in connective tissues and are not a normal, prominent resident of the bone marrow. They are significantly smaller than megakaryocytes.

• a) Myeloblast: This is the earliest recognizable stage. The cytoplasm is basophilic and contains no granules.

• b) Promyelocyte: This stage is characterized by the production of azurophilic (primary) granules. These are non-specific granules that contain myeloperoxidase and other bactericidal enzymes.

• c) Myelocyte: This is the stage where specific (secondary) granules first appear. These granules are specific to the cell lineage (neutrophilic, eosinophilic, or basophilic). For neutrophils, these specific granules contain lactoferrin and collagenase. The appearance of these specific granules defines the shift from the promyelocyte to the myelocyte stage. This is the last stage capable of cell division.

• d) Metamyelocyte: At this stage and beyond, the cell no longer divides and only undergoes nuclear maturation (indentation and segmentation). Granule production has ceased.

 Erythropoietin: It is produced primarily by the kidneys in response to low oxygen levels in the blood (hypoxia). It travels to the bone marrow and stimulates the stem cells to develop into mature red blood cells. This increases the oxygen-carrying capacity of the blood.

Why not the others?

• a) Thrombopoietin (TPO): Stimulates megakaryocyte and platelet production, not RBCs.

• c) Interleukin-6 (IL-6): A cytokine involved in inflammation and stimulating hepcidin (iron regulation), not a direct RBC stimulator.

• d) Granulocyte colony-stimulating factor (G-CSF): Stimulates neutrophil production, not RBCs.

Platelets (thrombocytes) are small, disk-shaped cell fragments that play a key role in blood clotting by adhering to damaged blood vessel walls, forming a platelet plug, and releasing factors that help in coagulation.

Why the other options are incorrect:

• a) Oxygen transport: This is the primary function of red blood cells via the hemoglobin they contain.

• b) Phagocytosis: This is the process of engulfing and destroying pathogens or debris. It is a key function of certain white blood cells, like neutrophils and macrophages, not platelets.

• d) Antibody production: Antibodies are produced by B lymphocytes (a type of white blood cell) as part of the adaptive immune response. Platelets have no role in this process.

Erythropoietin (EPO) is a hormone produced primarily by the kidneys, and its fundamental role is to stimulate the bone marrow to produce more red blood cells (RBCs). The primary trigger for its release is a low level of oxygen in the blood, a condition known as hypoxia.

Why the other options are incorrect:

• a) Polycythemia vera: This is a condition characterized by an excess production of red blood cells. However, this overproduction is independent of EPO; it is caused by a mutation in the bone marrow cells. In fact, EPO levels in polycythemia vera are typically low because the high RBC count carries enough oxygen, suppressing the normal stimulus for EPO production.

• c) Iron overload: Conditions like hemochromatosis involve excess iron storage. Iron is a raw material for making hemoglobin, but its availability does not directly regulate EPO production. The trigger for EPO is oxygen levels, not iron levels.

• d) Leukocytosis: This refers to an increased white blood cell count, often due to infection or inflammation. While some inflammatory cytokines can affect EPO production, leukocytosis itself is not a direct trigger for increased erythropoietin. The primary and direct trigger remains hypoxia.

• A normoblast (also called orthochromatic erythroblast) is the last stage of red blood cell development that still contains a nucleus.

• After this stage, the nucleus is extruded, and the cell becomes a reticulocyte, which is immature but anucleate.

• The reticulocyte then matures into a fully functional erythrocyte.

Why the other options are incorrect:

• a) Reticulocyte: The reticulocyte is not nucleated. It is the immediate post-nucleated stage.

• c) Pronormoblast: This is the first recognizable nucleated stage, not the last.

• d) Erythrocyte: The mature erythrocyte is the final, fully mature cell and has no nucleus.

Hematopoiesis is the process of forming new blood cells (red blood cells, white blood cells, and platelets).

• During fetal development, the liver and spleen are the primary sites of hematopoiesis.

• However, in a healthy adult, this process almost entirely shifts to the bone marrow, specifically the red bone marrow found in the pelvis, ribs, sternum, and vertebrae.

Let’s look at why the other options are incorrect:

• a) Liver: While it is a crucial primary site for the fetus, its hematopoietic function significantly decreases before birth and is not active in healthy adults.

• b) Spleen: The spleen acts as a site for hematopoiesis in the fetus. In adults, it is primarily involved in filtering blood, removing old or damaged red blood cells, and acting as a reservoir for platelets and white blood cells. It can resume hematopoiesis only in severe diseases, like myelofibrosis, where the bone marrow is compromised.

• d) Thymus: The thymus is not a site for general blood cell production. Its role is specific to the maturation and development of a particular type of white blood cell called T-lymphocytes (T-cells).

Cytoplasmic basophilia (a blue tint when stained with Romanowsky dyes like Wright-Giemsa) is caused by the presence of ribosomal RNA, which is involved in protein synthesis.

• Immature cells have high levels of RNA because they are actively producing proteins, including hemoglobin in the case of red cell precursors. This gives their cytoplasm a pronounced basophilic (blue) color.

• As a cell matures, it accumulates more hemoglobin (which is eosinophilic/pink) and its RNA content decreases. This leads to a more pinkish or polychromatophilic (mixed blue and pink) cytoplasm, and finally to the fully mature, salmon-pink color of a mature red blood cell.

The Donath-Landsteiner (DL) antibody is the specific, diagnostic autoantibody found in Paroxysmal Cold Hemoglobinuria (PCH).

Why the other options are wrong:

• a) IgM cold agglutinin: This describes the antibody in Cold Agglutinin Disease. It is IgM and causes agglutination (clumping) of red cells in the cold, not typically direct, intravascular hemolysis via complement in the same biphasic manner.

• b) Biphasic IgM hemolysin: While “biphasic” is correct, the antibody in PCH is IgG, not IgM.

• d) IgG warm agglutinin: This describes the more common antibodies found in Warm Autoimmune Hemolytic Anemia. These are IgG, but they react best at 37°C (warm) and primarily cause extravascular hemolysis in the spleen, not the characteristic biphasic, complement-mediated intravascular hemolysis of PCH.

Platelets contain different types of granules that are released during activation to facilitate hemostasis (clotting).

• a) Alpha granules: These are the most numerous platelet granules. Their contents are crucial for coagulation and clot formation. They contain:

  • Clotting Factors: Such as Factor V, Factor VIII, and von Willebrand Factor (vWF).

  • Fibrinogen: A key protein that is converted into fibrin to form the meshwork of a blood clot.

  • Other Proteins: Platelet-derived growth factor (PDGF), fibronectin, and thrombospondin.

Why the other options are incorrect:

• b) Dense granules: These granules contain small molecules that promote platelet activation and aggregation, but not clotting factors or fibrinogen. Their contents include ADP, ATP, calcium, and serotonin.

• c) Lysosomal granules: These contain hydrolytic enzymes (e.g., acid hydrolases) primarily for digesting internal and external debris. They are not primarily involved in the clotting cascade.

• d) Primary granules: This term is typically used to describe the granules found in neutrophils (myeloperoxidase, defensins), not platelets.

• Megakaryocytes are the large precursor cells in the bone marrow that give rise to platelets.

• In a normal bone marrow aspirate, when examined under low power (10x objective), the expected number of megakaryocytes is about 1–2 per low-power field (LPF).

Why not the others?

• 5–10, 15–20, or 25–30 per LPF → would suggest megakaryocytic hyperplasia, which is seen in conditions like myeloproliferative neoplasms (e.g., essential thrombocythemia, primary myelofibrosis, CML) or in reactive thrombocytosis.

• Eosinophils are white blood cells that combat multicellular parasites (like helminths) and play a key role in allergic reactions.

• They release cytotoxic granules and inflammatory mediators to attack parasites and modulate allergic inflammation.

Why the other options are incorrect:

• b) Basophils: Basophils are also involved in allergic reactions. They are the primary cells that release histamine, which causes immediate allergy symptoms like itching and hives. However, they are not the primary cell increased in parasitic infections. Eosinophils are the classic and most prominent cell elevated in both conditions.

• c) Neutrophils: Neutrophils are the first responders to bacterial and fungal infections, not typically parasites or allergens.

• d) Monocytes: Monocytes become macrophages in tissues and are involved in phagocytosing pathogens, clearing debris, and presenting antigens. They are not specifically elevated in allergic or parasitic responses.

A pluripotent hematopoietic stem cell (also known as a multipotent stem cell) is the most primitive and undifferentiated cell in the bone marrow. Its defining characteristic is the ability to give rise to all blood cell lineages.

Why the other options are incorrect:

• a) Daughter cells of only one cell line: This describes a committed progenitor cell (e.g., a myeloid progenitor), not a pluripotent stem cell.

• b) Only T lymphocytes and B lymphocytes: This describes a lymphoid progenitor, which is already a more differentiated cell.

• c) Erythropoietin, thrombopoietin, and leukopoietin: These are cytokines (hormones) that regulate hematopoiesis; they are produced by the kidneys, liver, and other tissues, not by the stem cells themselves.

Hemoglobin production during development changes with gestational age:

• Alpha (α) chains → Begin early in embryonic life (around 6 weeks) and remain constant throughout fetal and adult life.

• Gamma (γ) chains → Major component of fetal hemoglobin (HbF = α2γ2). High during fetal life, but production declines after birth.

• Beta (β) chains → Production is very low during early gestation, but begins to rise significantly after the 30th week of gestation, preparing for the switch from HbF → HbA (α2β2) around birth.

• Delta (δ) chains → Produced in small amounts, form HbA2 (α2δ2), but do not show a dramatic rise like beta chains.

• In a normal adult, the spleen’s main roles include:

  • Filtering blood and removing old, damaged, or abnormal red blood cells.

  • Immune functions (e.g., phagocytosis of bacteria, antibody production).

• a) Storage of RBCs – The spleen stores platelets and some RBCs, but this is not its primary role in humans (more significant in some other species).

• b) Production of RBCs – This occurs in the bone marrow after birth; the spleen may resume erythropoiesis only in severe hematologic stress.

• c) Synthesis of erythropoietin – This is mainly done by the kidneys in adults.

• Monocytes are the largest white blood cells in peripheral blood, with abundant cytoplasm and a kidney-shaped nucleus.

• They circulate in the blood and differentiate into macrophages or dendritic cells in tissues, playing a key role in phagocytosis and antigen presentation.

Why the other options are incorrect:

• a) Neutrophil: Neutrophils are smaller than monocytes.

• c) Eosinophil: While eosinophils are large cells, they are generally slightly smaller than monocytes.

• d) Lymphocyte: Lymphocytes are typically the smallest of the white blood cells.

The average life span of a platelet in the circulation is approximately 7 to 10 days.

This is a standard value in hematology. The other options do not align with the typical platelet life span:

• a) 5 days is too short.

• c) 20 days and d) 30 days are far too long and are more characteristic of red blood cell survival (which is about 120 days).

• Von Willebrand factor (vWF) is a large glycoprotein involved in hemostasis.

• It plays a key role in platelet adhesion to subendothelium and serves as a carrier protein for Factor VIII in plasma.

Why not the other options?

• Myeloblasts → precursors of granulocytes; not involved in vWF synthesis.

• Monoblasts → precursors of monocytes/macrophages; do not produce vWF.

• Lymphoblasts → precursors of lymphocytes; unrelated to vWF synthesis.

• Erythrocytes (red blood cells) contain hemoglobin, which binds to oxygen in the lungs and releases it in the tissues, and also assists in transporting carbon dioxide (mostly as bicarbonate, and partly bound to hemoglobin).

• Granulocytes (a type of white blood cell) are involved in immune response, not gas transport.

• Lymphocytes (another type of white blood cell) are involved in adaptive immunity.

• Thrombocytes (platelets) function in clotting.

The immune system is broadly divided into two branches: the innate immune response and the adaptive (or acquired) immune response.

• b) Lymphocytes: These are the primary cells of the adaptive immune response. This system is characterized by its specificity and memory. Lymphocytes can recognize and remember specific pathogens, providing long-lasting immunity.

Why the other options are incorrect:

• a) Neutrophils: These are key players in the innate immune response. They provide a rapid, non-specific defense against pathogens, especially bacteria, but do not confer long-term immunity.

• c) Monocytes/Macrophages: These cells are also part of the innate immune system. They phagocytose (engulf) pathogens and present antigens to lymphocytes, thus acting as a crucial bridge between the innate and adaptive systems, but they are not the primary mediators of adaptive immunity themselves.

• d) Basophils: These cells are involved in innate immunity, particularly in allergic reactions and defense against parasites, by releasing histamine and other inflammatory mediators. They play no direct role in the specific, memory-based adaptive response.

• Methemoglobin is the form of hemoglobin where the iron (Fe) is in the ferric state (Fe³⁺) instead of the normal ferrous state (Fe²⁺). This form cannot bind oxygen effectively.

• Carboxyhemoglobin is hemoglobin bound to carbon monoxide.

• Sulfhemoglobin is a rare, green-pigmented form of hemoglobin resulting from the incorporation of sulfur.

• Ferrihemoglobin is a chemically accurate synonym for methemoglobin, but “methemoglobin” is the standard and widely accepted clinical and physiological term.

The maturation of megakaryoblasts into megakaryocytes is a unique process called endomitosis.

• In endomitosis, the cell’s DNA replicates (nuclear division), but the cell itself does not undergo cytokinesis (the splitting of the cytoplasm).

• This results in a single, large cell with a single, large and multilobulated nucleus containing a polyploid amount of DNA (e.g., 8N, 16N, 32N, or even 64N).

• While the nucleus is replicating, the cytoplasm matures continuously—it does not divide. It becomes more abundant and develops the demarcation membrane system that will eventually fragment to form platelets.

Why the other options are incorrect:

• a) Progressive decrease in overall cell size: The opposite occurs. Megakaryocytes become some of the largest cells in the bone marrow.

• b) Increasing basophilia of cytoplasm: The cytoplasm actually becomes less basophilic (less blue) and more granular as it matures.

• d) Fusion of the nuclear lobes: The nuclear lobes are part of a single, highly lobulated nucleus; they do not form from the fusion of separate nuclei.

• Primary (central) lymphoid organs are where lymphocytes develop and mature:

  • Bone marrow → site of B cell development (and all blood cell precursors).

  • Thymus → site of T cell maturation.

Why the other options are incorrect:

• Spleen → secondary lymphoid organ; filters blood and mounts immune responses.

• Lymph nodes → secondary lymphoid organ; filter lymph and activate immune cells.

• Tonsils → secondary lymphoid tissue; trap pathogens from oral/nasal cavities.

Hemoglobin’s oxygen affinity increases in alkaline conditions (higher pH). This is known as the Bohr effect:

• High pH → less H⁺ → hemoglobin holds oxygen more tightly

• Low pH (acidosis), high CO₂, high temperature, and high 2,3-BPG → decrease O₂ affinity, promoting oxygen release to tissues.

Why the other options are incorrect:

• a) Increased 2,3-BPG (2,3-Bisphosphoglycerate): This compound binds to deoxyhemoglobin, stabilizing it and decreasing oxygen affinity. Levels of 2,3-BPG increase in conditions like chronic hypoxia (e.g., at high altitudes) to facilitate oxygen delivery.

• c) Increased CO₂: Carbon dioxide reacts with water to form carbonic acid, which dissociates and lowers the pH. This decrease in pH (acidosis) decreases oxygen affinity (part of the Bohr effect).

• d) Increased temperature: Higher temperature, often associated with fever or exercising muscles, decreases oxygen affinity, ensuring that more oxygen is released to the active, metabolizing tissues that need it.

• In normal, functional hemoglobin, the iron in the heme group is in the ferrous state (Fe²⁺). This allows it to bind oxygen (O₂).

• Methemoglobin is the dysfunctional form of hemoglobin where this iron has been oxidized to the ferric state (Fe³⁺).

• Ferric iron (Fe³⁺) cannot bind oxygen. The presence of methemoglobin therefore reduces the oxygen-carrying capacity of the blood.

• About 65–70% of total body iron is found in hemoglobin within red blood cells, where it is essential for oxygen transport.

Why the other options are incorrect:

• a) Ferritin: Ferritin is the primary iron storage protein. It stores about 20–25% of the body’s iron, mainly in the liver, spleen, and bone marrow, providing a readily mobilizable reserve.

• b) Transferrin: Transferrin is an iron transport protein in the blood plasma. It binds to iron and transports it through the bloodstream. However, the amount of iron actually bound to transferrin at any given time is very small, constituting only about 0.1% of total body iron.

• d) Hemosiderin: Hemosiderin is another, less readily available, form of iron storage. It is an insoluble protein-iron complex that accumulates when iron stores are high, making up a variable but smaller percentage than ferritin.

The breakdown of hemoglobin after red blood cells are removed from circulation (primarily in the spleen, liver, and bone marrow) follows these initial key steps:

1. The hemoglobin molecule is first split into its heme and globin components.

2. The globin protein chain is further broken down into its constituent amino acids, which are recycled by the body.

3. The heme portion is then broken down into iron and biliverdin (a green pigment).

Therefore, the primary products of the initial breakdown are iron, heme, and globin, with the heme and globin undergoing further catabolism.

Why the other options are incorrect:

• a) Iron, porphyrin, and amino acids: The porphyrin ring is part of the heme molecule, but heme is the immediate product, not the free porphyrin.

• b) Heme, protoporphyrin, and amino acids: Protoporphyrin is the precursor to heme in synthesis, not a direct breakdown product.

• d) Heme, hemosiderin, and globin: Hemosiderin is a storage form of iron that can be formed later from the released iron, but it is not an immediate, direct breakdown product of hemoglobin.

• Interleukin-5 (IL-5) is the primary cytokine responsible for the growth, differentiation, activation, and survival of eosinophils. It is the key regulator of eosinophil production and release from the bone marrow into the bloodstream.

Why the other options are incorrect:

• a) IL-1: Primarily involved in inflammation and fever; not a major regulator of eosinophils.

• b) IL-2: A key growth factor for T-lymphocytes.

• c) IL-4: Important for the differentiation of T-helper 2 (Th2) cells and for B-cell class switching to IgE, which can stimulate eosinophil activity indirectly, but it is not the primary cytokine for eosinophil production like IL-5.

• Basophils (and their tissue counterparts, mast cells) are the primary cells involved in immediate (type I) hypersensitivity reactions (e.g., allergies, anaphylaxis).

• They have high-affinity IgE receptors on their surface, and upon exposure to an allergen, they degranulate and release histamine, leukotrienes, and other mediators that cause rapid symptoms.

• Eosinophils are involved in later stages of allergic responses and defense against parasites, but not the primary mediators of the immediate reaction.

• Reactive lymphocytes are involved in adaptive immune responses (viral, delayed hypersensitivity), not immediate hypersensitivity.

• Plasma cells produce antibodies but do not release immediate allergic reaction mediators

HbF (fetal hemoglobin) is the main hemoglobin during fetal life, with higher oxygen affinity than adult hemoglobin (HbA), facilitating oxygen transfer from maternal blood.

Why the other options are incorrect:

• a) HbA (α₂β₂): This is the predominant adult hemoglobin, making up about 95-98% of hemoglobin in a healthy adult. It is present in only very small amounts in the fetus.

• b) HbA₂ (α₂δ₂): This is a minor adult hemoglobin, typically making up about 2-3% of the total in adults. It is not a significant fetal hemoglobin.

• d) HbC: This is an abnormal hemoglobin variant caused by a mutation. It is not a normal fetal or adult hemoglobin and is associated with hemoglobin C disease, a mild hemolytic anemia.

In a normal adult’s peripheral blood (the blood circulating in the bloodstream), neutrophils are the most abundant type of white blood cell, making up approximately 40-60% of the total white blood cell count.

Why the other options are incorrect:

• b) Lymphocytes: While they are the second most common white blood cell, they are not the most abundant in the blood under normal conditions. They are, however, the predominant cell type in the lymphatic system.

• c) Monocytes: These are much less common, typically comprising less than 10% of the white blood cell count.

• d) Eosinophils: These are even less common, usually making up only 1-4% of the total.

• Erythropoietin (EPO) is a hormone produced mainly by the kidneys in response to hypoxia.

• Its primary action is to stimulate erythropoiesis in the bone marrow by acting on committed erythroid progenitor cells (CFU-E and later stages).

• It promotes cell survival, proliferation, and differentiation of red blood cell precursors, partly by stimulating RNA and protein synthesis necessary for hemoglobin production and maturation.

• It also increases the release of reticulocytes from the bone marrow (opposite of option d).

The other options are incorrect because:

• a) EPO does not primarily affect granulocyte replication.

• c) Colony-stimulating factors for other cell lines are not the main target of EPO.

• d) EPO increases, not decreases, marrow reticulocyte release.

Release of Histamine: This is the primary function of basophils. They are packed with granules containing histamine and other inflammatory mediators like heparin. When activated (often by allergens or parasites), they release these substances. Histamine causes the classic symptoms of inflammation and allergy, such as vasodilation (leading to redness and swelling), increased vascular permeability, and smooth muscle contraction.

Why the other options are incorrect:

• a) Phagocytosis: This is not a major function of basophils. Phagocytosis (engulfing and destroying pathogens) is the primary role of neutrophils and macrophages.

• b) Antibody Production: Antibodies are produced by B lymphocytes (B cells), which are a completely different type of white blood cell. Basophils do not produce antibodies.

• d) Killing of Parasites: While basophils are involved in the immune response against parasites, they are not the primary killer cells. They contribute by recruiting and activating other cells, particularly eosinophils, which are the main white blood cells responsible for directly attacking and killing parasites.

• Granulocytes (specifically neutrophils and, to a lesser extent, eosinophils) are specialized phagocytic cells in the immune system that engulf and destroy pathogens.

• Lymphocytes are mainly involved in adaptive immunity (antibody production, cell-mediated responses) but are not primarily phagocytic.

• Erythrocytes (red blood cells) carry oxygen and do not perform phagocytosis.

• Thrombocytes (platelets) are involved in clotting and wound healing, not phagocytosis.

Spherocytes are abnormal, spherical red blood cells that have lost their characteristic biconcave disc shape. They form due to a loss of membrane surface area without a proportional loss of cytoplasm (hemoglobin).

Why the other options are incorrect:

• b) Cellular membrane proteins: This is the direct cause. The most common deficiency is in spectrin, ankyrin, or other proteins that make up the cytoskeletal network underlying the red cell membrane. This defective structural scaffolding makes the membrane unstable. As these unstable pieces of the membrane are pinched off and removed by the spleen, the cell loses surface area. The cell then remodels into the smallest possible shape for its volume—a sphere. This is the hallmark of Hereditary Spherocytosis.

In healthy adults, neutrophils are the most abundant type of white blood cell, making up about 40–60% of circulating leukocytes. They are the first responders to bacterial infections and play a central role in acute inflammation.

Why the other options are incorrect:

• a) 10–20%: This range is far too low for neutrophils. A neutrophil count this low is called severe neutropenia and significantly increases the risk of life-threatening infection.

• b) 20–40%: This is the typical range for lymphocytes, not neutrophils. If neutrophils were in this range, it would often indicate a low count (neutropenia) or a relative increase in lymphocytes.

• d) 70–90%: This range is too high. A neutrophil percentage this elevated is called neutrophilia and is typically a sign of a acute bacterial infection, inflammation, or other stressor on the body.

Red blood cells (RBCs) are particularly vulnerable to oxidative damage because they carry oxygen, which can generate reactive oxygen species (free radicals). They lack a nucleus and mitochondria, so their ability to repair themselves is limited.

Why the other options are incorrect:

• a) Amylase: This is a digestive enzyme that breaks down starch. It has no role in RBC physiology or oxidative protection.

• b) Catalase: This enzyme does help protect cells by breaking down hydrogen peroxide into water and oxygen. While it is present in RBCs, the NADPH/glutathione system (dependent on G6PD) is considered the primary and most crucial defense mechanism against a wider range of oxidative stressors.

• d) Lipase: This is a digestive enzyme that breaks down fats. It has no role in protecting RBCs.

• Hemoglobin binds oxygen reversibly in the heme group, where the iron must be in the ferrous (Fe²⁺) form.

• If it oxidizes to the ferric (Fe³⁺) state, hemoglobin becomes methemoglobin, which cannot bind oxygen reversibly.

• Haptoglobin binds free hemoglobin in plasma (not relevant to O₂ binding).

• Iron is not free in the cytoplasm; it’s bound in heme.

• Monocytes circulate in the blood and, upon entering tissues, differentiate into macrophages or dendritic cells.

• Macrophages are phagocytic cells that engulf pathogens, dead cells, and debris, playing a key role in innate immunity and tissue repair.

Auer rods are needle-shaped, cytoplasmic inclusions that are pathognomonic for a myeloid malignancy, most commonly Acute Myeloid Leukemia (AML).

• They are formed by the fusion of azurophilic (primary) granules. These granules contain crystalline arrays of enzymes like myeloperoxidase.

• This abnormal fusion creates the sharp, rod-like appearance visible under a light microscope.

• The presence of Auer rods is a critical diagnostic finding as it definitively classifies a leukemia as being of myeloid, not lymphoid, origin.

Why the other options are incorrect:

• a) DNA precipitates: Nuclear material is not the composition of Auer rods.

• c) Denatured hemoglobin: This describes Heinz bodies, found in red blood cells.

• d) Large cytoplasmic granules: This is too vague. While Auer rods are a type of granule, their specific composition and defining feature is that they are fused primary (azurophilic) granules.

Normal adult hemoglobin (HbA) is a tetramer composed of two alpha chains and two beta chains (α₂β₂).

Let’s look at the role of each chain:

• a) Alpha: Present in HbA. The alpha-globin chain is a core component of most human hemoglobins throughout life, including fetal hemoglobin (HbF) and adult hemoglobin (HbA).

• b) Beta: Present in HbA. The beta-globin chain is the other core component that makes up the majority (~97%) of hemoglobin in a healthy adult.

• c) Gamma: Absent in HbA. The gamma-globin chain is the primary component of fetal hemoglobin (HbF), which has the structure α₂γ₂. HbF is the main hemoglobin type during the last seven months of fetal development and the first few months after birth. After birth, production of gamma chains decreases dramatically and is replaced by beta chain production. Therefore, gamma chains are not a part of normal adult hemoglobin.

• d) Delta: Largely absent, but a minor form exists. The delta-globin chain is a component of Hemoglobin A2 (HbA2), which has the structure α₂δ₂. HbA2 is a normal minor adult hemoglobin, but it is present only in very small amounts (about 2-3% of total hemoglobin). Since the question specifies the primary “normal adult hemoglobin (HbA),” the gamma chain is the one that is truly absent.

• A normal red blood cell (RBC) circulates in the bloodstream for about 120 days (≈ 4 months).

• After this period, the RBCs lose membrane flexibility and are removed by splenic macrophages (reticuloendothelial system).

• Hemoglobin is then broken down:

  • Iron → recycled.

  • Globin → broken into amino acids.

  • Heme → converted to bilirubin.

Why not the others?

• 60 or 90 days → too short; seen in certain hemolytic anemias, not normal physiology.

• 150 days → longer than the normal survival period.

The spleen acts as the “graveyard of red blood cells.” It removes old or damaged erythrocytes through macrophages in the red pulp. Hemoglobin is broken down, iron is recycled, and bilirubin is sent to the liver for processing.

Why the other options are incorrect:

• a) Kidney: The kidneys are not primarily involved in removing RBCs from circulation. Their role is to filter waste from the blood to form urine. While they can filter out free hemoglobin if RBCs break down within the bloodstream (intravascular hemolysis), this is not the primary or normal pathway for removing aged RBCs.

• b) Liver: The liver does contain macrophages (Kupffer cells) that can remove old or damaged RBCs, and it plays a significant secondary role. However, the spleen is the more efficient and primary site for this daily “culling” process due to its specialized slow-flow circulation and structure.

• d) Bone marrow: The bone marrow is the site of red blood cell production (hematopoiesis), not destruction.

• Each heme group in hemoglobin contains a ferrous iron (Fe²⁺) atom, which directly binds oxygen.

• Hemoglobin has 4 heme groups, so each molecule can carry up to 4 oxygen molecules.

Why the other options are incorrect:

• a) Globin chain: The globin chains (alpha and beta) form the protein structure that surrounds and protects the heme group. They create a hydrophobic pocket that prevents the iron from being oxidized (changing to Fe³⁺, which cannot bind oxygen). While crucial for the molecule’s function and stability, the globin chains themselves do not bind the oxygen.

• c) Heme porphyrin ring: The porphyrin ring is a large, organic, ring-shaped structure that holds the iron atom in its center. Its role is to provide the stable platform for the iron. The ring itself does not bind the oxygen.

• d) 2,3-BPG (2,3-Bisphosphoglycerate): This molecule does not bind oxygen. Instead, it binds to a specific site on the deoxyhemoglobin form, stabilizing it and decreasing hemoglobin’s affinity for oxygen. This is a crucial regulatory mechanism that facilitates the release of oxygen to the tissues.

• The hexose monophosphate shunt (also called the pentose phosphate pathway) in erythrocytes generates NADPH.

• NADPH is used to maintain glutathione in its reduced form (GSH).

• Reduced glutathione protects hemoglobin and the cell from oxidative damage by neutralizing peroxides and free radicals.

• This prevents oxidation of heme iron (Fe²⁺ to Fe³⁺), which would form methemoglobin and impair oxygen transport.

The other options:

• a) 2,3-DPG regulation occurs in the glycolytic pathway (Rapoport-Luebering shunt), not HMP shunt.

• b) Energy (ATP) comes from glycolysis, not mainly the HMP shunt.

• d) The HMP shunt prevents oxidation of heme iron, not reduction.

• MCV measures the average volume or size of red blood cells.

  • Low MCV → microcytic anemia

  • High MCV → macrocytic anemia

Why the other options are incorrect:

• b) MCH (Mean Corpuscular Hemoglobin): This measures the average mass (weight) of hemoglobin per red blood cell. It does not indicate size directly.

• c) MCHC (Mean Corpuscular Hemoglobin Concentration): This measures the average concentration of hemoglobin in a given volume of red blood cells. It indicates how “full” of hemoglobin the cells are, not their physical size.

• d) RDW (Red Cell Distribution Width): This is a measure of the variation in size (volume) among the red blood cells, indicating the degree of anisocytosis (variation in cell size). It is not the average size itself.

The kidneys are the primary site of erythropoietin (EPO) production in adults. They have specialized cells, called peritubular interstitial fibroblasts in the renal cortex, that are highly sensitive to blood oxygen levels.

Why the other options are incorrect:

• a) Liver: The liver is the primary site of EPO production in the fetus. After birth, it takes on a minor, backup role (producing about 10-15% of EPO), but the kidneys are the dominant producer in adults.

• c) Spleen: The spleen’s main functions are filtering blood, removing old RBCs, and serving as a site for immune activity. It does not produce erythropoietin.

• d) Pancreas: The pancreas is an organ of the digestive and endocrine systems, responsible for producing digestive enzymes and hormones like insulin and glucagon. It has no role in erythropoietin production.

• Neutrophils are the most abundant WBCs (50–70% of circulating leukocytes) and are the first responders to sites of bacterial infection.

• They act as the first line of defense through:

  • Phagocytosis of bacteria.

  • Release of enzymes and reactive oxygen species to kill pathogens.

  • Formation of neutrophil extracellular traps (NETs).

Why not the others?

• Lymphocytes → key players in adaptive immunity (B-cells & T-cells), not immediate bacterial defense.

• Monocytes → differentiate into macrophages, arrive later for chronic infection and cleanup.

• Basophils → involved in allergic reactions and release histamine, not antibacterial defense.

About 65–70% of the body’s iron is found in hemoglobin within red blood cells, where it plays a key role in oxygen transport. Smaller amounts are stored as ferritin/hemosiderin, bound to transferrin in plasma, or present in myoglobin.

Hemoglobin is the iron-containing protein found in red blood cells, and its primary, essential function is to transport oxygen from the lungs to the tissues throughout the body.

Why the other options are incorrect:

• a) Carbon dioxide: While hemoglobin does play a role in carbon dioxide transport, it is not its primary function. Most carbon dioxide (about 70%) is transported in the blood plasma as bicarbonate ions (HCO₃⁻). A smaller portion (about 20-25%) is carried bound to hemoglobin, but to a different site than oxygen (it binds to the globin protein chains, forming carbaminohemoglobin). Oxygen transport remains the key role of hemoglobin.

• c) Nitrogen: Nitrogen is an inert gas in the context of human respiration and is not metabolically transported by hemoglobin.

• d) Glucose: Glucose is transported dissolved in the blood plasma, not by hemoglobin.