Blood and Immunity — MCQs

A 15-year-old girl incurs a cut to the sole of her foot after stepping on a piece of broken glass. On examination, a superficial 0.5-cm laceration ceases to bleed within 5 minutes after application of local pressure. Which of the following substances is released by endothelium and is most likely to counteract platelet aggregation near this site of injury?

Q377Medium

Hemoglobin's affinity for oxygen is increased due to which of the following factors?

Q378Easy

Deficiency of the intrinsic factor of Castle causes which of the following?

Q379Easy

Which factor is essential for the prothrombin time test?

Q380Easy

What are the primary sites of lymphocyte production?

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 371: Which of the following decreases the affinity of oxygen with hemoglobin?

A. Decreased H+ ions
B. 2,3 BPG (Correct Answer)
C. Decrease in temperature
D. Decreased sorbitol

Explanation: **Explanation:** The affinity of hemoglobin (Hb) for oxygen is represented by the **Oxygen-Dissociation Curve (ODC)**. A **rightward shift** in this curve indicates a decrease in oxygen affinity, meaning Hb releases oxygen more easily to the tissues. **1. Why 2,3-BPG is correct:** 2,3-Bisphosphoglycerate (2,3-BPG) is a byproduct of glycolysis in red blood cells. It binds to the central cavity of the deoxyhemoglobin tetramer, stabilizing the **T-state (Tense state)** of hemoglobin. This stabilization reduces Hb's affinity for oxygen, promoting oxygen unloading in peripheral tissues. Levels of 2,3-BPG increase during chronic hypoxia, high altitude, and anemia. **2. Analysis of Incorrect Options:** * **A. Decreased H+ ions (Alkalosis):** A decrease in H+ (increase in pH) causes a **leftward shift** (Bohr effect), increasing oxygen affinity and making it harder for tissues to receive oxygen. * **C. Decrease in temperature:** Hypothermia stabilizes the bond between oxygen and hemoglobin, shifting the curve to the **left** (increased affinity). * **D. Decreased sorbitol:** Sorbitol levels are related to glucose metabolism (polyol pathway) and do not directly influence the oxygen-hemoglobin dissociation curve. **3. NEET-PG High-Yield Pearls:** * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis (H+), **D**PG (2,3-BPG) increase, **E**xercise, **T**emperature increase. * **Fetal Hemoglobin (HbF):** Has a **higher affinity** for oxygen (Left shift) because it binds 2,3-BPG poorly due to the presence of gamma chains instead of beta chains. * **P50:** The partial pressure of oxygen at which Hb is 50% saturated. A **right shift increases the P50**.

Question 372: Which test is used for the detection of the intrinsic pathway of coagulation?

A. Activated Partial Thromboplastin Time (aPTT) (Correct Answer)
B. Prothrombin Time (PT)
C. Bleeding Time (BT)
D. Clotting Time (CT)

Explanation: **Explanation:** The coagulation cascade is divided into the intrinsic, extrinsic, and common pathways. The **Activated Partial Thromboplastin Time (aPTT)** is the specific screening test used to evaluate the **intrinsic pathway** (Factors XII, XI, IX, VIII) and the **common pathway** (Factors X, V, II, I). It measures the time taken for a fibrin clot to form after adding an activator (like kaolin or silica) and phospholipids to the plasma. **Analysis of Options:** * **Prothrombin Time (PT):** This tests the **extrinsic pathway** (Factor VII) and the common pathway. It is the investigation of choice for monitoring Warfarin therapy and assessing liver function. * **Bleeding Time (BT):** This measures **primary hemostasis**, specifically platelet function and vascular integrity, rather than the coagulation cascade. It is prolonged in conditions like Von Willebrand Disease and Thrombocytopenia. * **Clotting Time (CT):** While it measures the time for blood to clot, it is a crude, non-specific bedside test (e.g., Lee-White method) that lacks the sensitivity and standardization of aPTT. **High-Yield Clinical Pearls for NEET-PG:** * **Heparin Monitoring:** aPTT is the gold standard for monitoring unfractionated heparin therapy. * **Hemophilia:** Both Hemophilia A (Factor VIII deficiency) and Hemophilia B (Factor IX deficiency) result in an **isolated prolonged aPTT** with a normal PT and BT. * **Vitamin K Deficiency:** Initially prolongs PT (due to the short half-life of Factor VII) before affecting aPTT. * **Mixing Studies:** If aPTT is prolonged, a mixing study is done; correction indicates factor deficiency, while non-correction suggests an inhibitor (e.g., Lupus anticoagulant).

Question 373: Serotonin is secreted by which of the following?

A. Leukocytes
B. RBCs
C. Platelets (Correct Answer)
D. Vessel wall

Explanation: **Explanation:** **Correct Answer: C. Platelets** **Medical Concept:** Serotonin (5-hydroxytryptamine) is a potent vasoconstrictor and signaling molecule. While it is synthesized primarily in the enterochromaffin cells of the gastrointestinal tract, it is not synthesized by platelets. Instead, **platelets actively take up serotonin** from the plasma via the serotonin transporter (SERT) and store it in high concentrations within their **dense granules (delta granules)**. During platelet activation and the subsequent release reaction, serotonin is secreted into the local environment to promote vasoconstriction and enhance platelet aggregation, playing a crucial role in primary hemostasis. **Why other options are incorrect:** * **Leukocytes (WBCs):** While some leukocytes (like basophils) release histamine and heparin, they are not the primary storage or secretory sites for serotonin in the blood. * **RBCs:** Red blood cells are specialized for gas transport (Oxygen/CO2) and do not possess the machinery or granules to store or secrete serotonin. * **Vessel Wall:** The vascular endothelium produces substances like Nitric Oxide (NO) and Prostacyclin ($PGI_2$) for vasodilation, and Endothelin-1 for vasoconstriction, but it does not secrete serotonin. **High-Yield Clinical Pearls for NEET-PG:** * **Platelet Granules:** * **Dense (Delta) Granules:** Contain **SAC** (Serotonin, ADP/ATP, Calcium). * **Alpha Granules:** Contain vWF, Fibrinogen, Platelet Factor 4, and PDGF. * **Serotonin Function:** In the blood, it acts as a vasoconstrictor to limit blood loss; in the CNS, it acts as a neurotransmitter regulating mood and sleep. * **Carcinoid Syndrome:** A clinical condition where neuroendocrine tumors secrete excessive serotonin, leading to flushing, diarrhea, and right-sided heart failure.

Question 374: Hemagglutinin (Anti-A and Anti-B) are which type of antibodies?

A. IgG
B. IgM (Correct Answer)
C. IgA
D. IgE

Explanation: **Explanation:** The correct answer is **B. IgM**. **Why IgM is correct:** Hemagglutinins (Anti-A and Anti-B) are naturally occurring antibodies found in the plasma of individuals who lack the corresponding antigens on their red blood cells. These are primarily **IgM** antibodies. Because IgM is a large pentameric molecule, it is highly efficient at agglutination (clumping) and activating the classical complement pathway. Crucially, due to their high molecular weight, IgM antibodies **cannot cross the placenta**, which is why ABO incompatibility between mother and fetus rarely causes severe Hemolytic Disease of the Newborn (HDN) in the same way Rh incompatibility does. **Why the other options are incorrect:** * **IgG:** While some "immune-type" Anti-A or Anti-B (seen after sensitization) can be IgG, the standard naturally occurring hemagglutinins are IgM. IgG is the only antibody that crosses the placenta. * **IgA:** These are primarily found in secretions (tears, saliva, colostrum) and provide mucosal immunity. They are not the primary antibodies responsible for ABO blood group reactions. * **IgE:** These antibodies are involved in Type I hypersensitivity reactions (allergies) and defense against parasitic infections. **High-Yield Clinical Pearls for NEET-PG:** * **Cold Agglutinins:** Naturally occurring ABO antibodies are "cold antibodies," meaning they react best at temperatures below 37°C (usually 4°C). * **Landsteiner’s Law:** States that if an agglutinogen (antigen) is present on RBCs, the corresponding agglutinin (antibody) must be absent from the plasma. * **Universal Donor/Recipient:** Type O individuals are universal donors (no A/B antigens); Type AB individuals are universal recipients (no Anti-A/Anti-B antibodies). * **Development:** Anti-A and Anti-B antibodies are not present at birth; they develop within 3–6 months of life due to exposure to similar antigens in gut bacteria and food.

Question 375: Which of the following is not useful for coagulation?

A. Plasmin (Correct Answer)
B. Thrombin
C. Calcium
D. Fibrin

Explanation: **Explanation:** The core concept tested here is the distinction between **coagulation (clot formation)** and **fibrinolysis (clot dissolution)**. **Why Plasmin is the correct answer:** Plasmin is a proteolytic enzyme whose primary function is **fibrinolysis**. It is the active form of plasminogen, activated by Tissue Plasminogen Activator (tPA). Plasmin works by degrading fibrin mesh into Fibrin Degradation Products (FDPs) and D-dimers. Therefore, it does not assist in coagulation; rather, it acts to dissolve an existing clot to prevent vessel occlusion. **Why the other options are incorrect:** * **Thrombin (Factor IIa):** This is the most critical enzyme in the coagulation cascade. It converts soluble fibrinogen into insoluble fibrin monomers and activates Factors V, VIII, XI, and XIII. * **Calcium (Factor IV):** Calcium ions are essential cofactors for almost all steps of the coagulation pathway (except the initial stages of the intrinsic pathway). Without calcium, blood cannot clot, which is why chelating agents like EDTA or Citrate are used in blood vials to prevent clotting. * **Fibrin (Factor Ia):** Fibrin is the end-product of the coagulation cascade. It forms the physical "mesh" or polymer that traps platelets and blood cells to create a stable hemostatic plug. **High-Yield NEET-PG Pearls:** * **D-Dimer:** A specific degradation product of cross-linked fibrin, used clinically to rule out DVT and PE. * **Antidote for Plasmin overactivity:** Tranexamic acid (TXA) or Epsilon-aminocaproic acid (EACA) inhibits plasminogen activation. * **Vitamin K-dependent factors:** II, VII, IX, and X (Calcium is required for these factors to bind to phospholipid surfaces).

Question 376: A 15-year-old girl incurs a cut to the sole of her foot after stepping on a piece of broken glass. On examination, a superficial 0.5-cm laceration ceases to bleed within 5 minutes after application of local pressure. Which of the following substances is released by endothelium and is most likely to counteract platelet aggregation near this site of injury?

A. Glycoprotein IIb/IIIa
B. Platelet-activating factor
C. Prostacyclin (Correct Answer)
D. Tissue-type plasminogen activator

Explanation: ### Explanation The correct answer is **Prostacyclin (PGI₂)**. **Why it is correct:** In the event of vascular injury, the goal of the body is to form a localized clot to stop bleeding (hemostasis) while preventing the clot from spreading to healthy, uninjured vessel segments. **Prostacyclin (PGI₂)** is a prostaglandin synthesized by healthy endothelial cells. It acts as a potent **vasodilator** and **inhibitor of platelet aggregation**. It increases intracellular cAMP levels in platelets, effectively counteracting the effects of Thromboxane A₂ (TXA₂), thereby ensuring the platelet plug remains localized only to the site of injury. **Analysis of Incorrect Options:** * **A. Glycoprotein IIb/IIIa:** This is a surface receptor found on platelets. When activated, it binds to fibrinogen to facilitate **platelet aggregation** (clumping). It promotes clot formation rather than counteracting it. * **B. Platelet-activating factor (PAF):** This is a potent phospholipid mediator derived from various cells (including neutrophils and endothelium) that **induces** platelet aggregation and degranulation. * **D. Tissue-type plasminogen activator (tPA):** While released by the endothelium, tPA is involved in **fibrinolysis** (breaking down an existing fibrin clot) by converting plasminogen to plasmin. It does not directly inhibit the initial stage of platelet aggregation. **NEET-PG High-Yield Pearls:** * **The Balance:** Hemostasis is a balance between **TXA₂** (from platelets; causes vasoconstriction/aggregation) and **PGI₂** (from endothelium; causes vasodilation/anti-aggregation). * **Aspirin Mechanism:** Low-dose aspirin irreversibly inhibits COX-1, shifting the balance in favor of PGI₂ because endothelial cells can regenerate the enzyme while platelets cannot. * **Nitric Oxide (NO):** Along with PGI₂, NO is another major substance released by healthy endothelium to inhibit platelet adhesion and promote vasodilation.

Question 377: Hemoglobin's affinity for oxygen is increased due to which of the following factors?

A. An increase in carbon dioxide levels
B. A rise in body temperature
C. A higher hemoglobin concentration
D. A decrease in 2,3-DPG (Correct Answer)

Explanation: ### Explanation The affinity of hemoglobin (Hb) for oxygen is represented by the **Oxygen-Dissociation Curve (ODC)**. A shift to the **left** indicates increased affinity (Hb holds onto $O_2$ more tightly), while a shift to the **right** indicates decreased affinity (Hb releases $O_2$ more easily). **Why Option D is Correct:** **2,3-Diphosphoglycerate (2,3-DPG)** is a byproduct of glycolysis in RBCs that binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and promoting oxygen unloading. Therefore, a **decrease in 2,3-DPG** stabilizes the "R" (Relaxed) state, increasing Hb's affinity for oxygen and shifting the curve to the **left**. This is commonly seen in stored blood (where 2,3-DPG depletes) and fetal hemoglobin (which has low affinity for 2,3-DPG). **Why the Other Options are Incorrect:** * **A & B: Increase in $CO_2$ and Temperature:** Both factors shift the curve to the **right** (Bohr Effect). Increased metabolic activity (high heat and $CO_2$ production) signals the need for more oxygen, thus decreasing Hb affinity to facilitate unloading at the tissues. * **C: Higher Hemoglobin Concentration:** While total oxygen-carrying capacity increases with more Hb, the **affinity** (the percentage saturation at a given $PO_2$) remains unchanged. --- ### High-Yield NEET-PG Pearls: * **Left Shift (Increased Affinity):** ↓ Temp, ↓ $H^+$ (Alkalosis), ↓ $CO_2$, ↓ 2,3-DPG, and **HbF** (Fetal Hb). * **Right Shift (Decreased Affinity):** ↑ Temp, ↑ $H^+$ (Acidosis), ↑ $CO_2$, ↑ 2,3-DPG. Remember the mnemonic **"CADET, face Right!"** (**C**$O_2$, **A**cid, **D**PG, **E**xercise, **T**emp). * **$P_{50}$ Value:** The partial pressure of $O_2$ at which Hb is 50% saturated. Normal value is **26.7 mmHg**. A left shift **decreases** the $P_{50}$.

Question 378: Deficiency of the intrinsic factor of Castle causes which of the following?

A. Megaloblastic anemia
B. Pernicious anemia (Correct Answer)
C. Cooley's anemia
D. Aplastic anemia

Explanation: **Explanation:** **Pernicious anemia** is the correct answer because it is specifically defined as a type of vitamin B12 deficiency caused by the lack of **Intrinsic Factor (IF)**. Intrinsic factor is a glycoprotein secreted by the **parietal cells** of the gastric mucosa. It binds to dietary Vitamin B12 (cobalamin) in the duodenum, and this IF-B12 complex is essential for absorption in the **terminal ileum**. Without IF—often due to autoimmune destruction of parietal cells—B12 cannot be absorbed, leading to impaired DNA synthesis in red blood cells. **Analysis of Incorrect Options:** * **Megaloblastic Anemia:** While pernicious anemia is a *type* of megaloblastic anemia, "Megaloblastic" is a broad category that includes deficiencies of either Vitamin B12 or Folate. Pernicious anemia is the specific clinical diagnosis when the cause is lack of IF. * **Cooley’s Anemia:** This is another name for **Thalassemia Major**, a genetic defect in hemoglobin synthesis (beta-chain), not a nutritional or absorption issue. * **Aplastic Anemia:** This refers to bone marrow failure resulting in pancytopenia (reduction in RBCs, WBCs, and platelets), usually due to toxins, drugs, or idiopathic causes, rather than vitamin deficiencies. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Absorption:** Vitamin B12 is absorbed in the **terminal ileum**; Folate is absorbed in the **jejunum**. * **Schilling Test:** Historically used to diagnose the cause of B12 deficiency (though now largely replaced by antibody testing). * **Neurological Symptoms:** Unlike folate deficiency, B12 deficiency causes **Subacute Combined Degeneration (SCD)** of the spinal cord due to the accumulation of methylmalonic acid. * **Parietal Cells:** These cells secrete both Intrinsic Factor and Hydrochloric acid (HCl).

Question 379: Which factor is essential for the prothrombin time test?

A. Thromboplastin (Correct Answer)
B. Prothrombin
C. Fibrinogen
D. Fibrin

Explanation: **Explanation:** The **Prothrombin Time (PT)** test evaluates the **Extrinsic** and **Common pathways** of the coagulation cascade. To perform this test, citrated plasma is added to a reagent containing **Tissue Factor (Factor III)** and **Calcium ions**. This reagent is collectively known as **Thromboplastin**. 1. **Why Thromboplastin is correct:** Thromboplastin acts as the external trigger that activates Factor VII, initiating the extrinsic pathway. Without the addition of exogenous thromboplastin, the extrinsic pathway cannot be assessed in a laboratory setting. 2. **Why other options are incorrect:** * **Prothrombin (Factor II) and Fibrinogen (Factor I):** These are clotting factors already present in the patient's plasma. They are the *substrates* being tested, not the essential reagent added to initiate the reaction. * **Fibrin:** This is the end-product of the coagulation cascade. The PT test measures the time it takes for a fibrin clot to form; it is not a factor required to start the test. **High-Yield Clinical Pearls for NEET-PG:** * **PT/INR** is specifically used to monitor **Warfarin (Oral Anticoagulant)** therapy because Warfarin inhibits Vitamin K-dependent factors (II, VII, IX, X), and Factor VII has the shortest half-life. * **Normal PT Range:** 11–13.5 seconds. * **Prolonged PT** is seen in Vitamin K deficiency, liver disease, and DIC. * **Mnemonic:** "PET" – **P**T measures the **E**xtrinsic pathway and is used for **T**houmadin (Warfarin). Conversely, **aPTT** measures the **I**ntrinsic pathway and is used for **H**eparin.

Question 380: What are the primary sites of lymphocyte production?

A. Bone marrow and Thymus (Correct Answer)
B. Spleen and Lymph nodes
C. Lymph nodes and mucosa associated lymphoid tissue
D. Spleen and mucosa associated lymphoid tissue

Explanation: ### Explanation **1. Why Option A is Correct:** Lymphocytes are produced and undergo their initial maturation in the **Primary Lymphoid Organs**. These consist of the **Bone Marrow** and the **Thymus**. * **Bone Marrow:** It is the site of hematopoiesis where all lymphocyte precursors originate. It is also the site where **B-lymphocytes** complete their maturation (B for Bone Marrow). * **Thymus:** Pro-T cells migrate from the bone marrow to the thymus to undergo selection and maturation into functional **T-lymphocytes** (T for Thymus). Because these organs "generate" the functional lymphocyte pool from stem cells, they are the primary sites of production. **2. Why Other Options are Incorrect:** * **Options B, C, and D:** These list **Secondary Lymphoid Organs** (Spleen, Lymph nodes, MALT, Peyer's patches). These organs do not produce lymphocytes *de novo*. Instead, they serve as "battlegrounds" where mature lymphocytes reside, circulate, and encounter antigens to initiate an immune response. While lymphocytes may undergo clonal expansion (proliferation) here during an infection, the primary production and "education" occur in the bone marrow and thymus. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bursa of Fabricius:** In birds, B-cells mature here; in humans, the bone marrow is the functional equivalent. * **Thymic Involution:** The thymus is most active during childhood and undergoes fatty atrophy (involution) after puberty, though T-cell production continues at a low level. * **DiGeorge Syndrome:** A high-yield clinical correlation where thymic hypoplasia leads to profound T-cell deficiency. * **Null Cells:** Natural Killer (NK) cells are also produced in the bone marrow and do not require the thymus for maturation.

Practice by Chapter

Composition and Functions of Blood

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Erythrocytes and Hemoglobin

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Leukocytes and Immune Function

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Platelets and Hemostasis

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Blood Groups and Transfusion

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Coagulation and Fibrinolysis

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Hematopoiesis

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Innate Immunity

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Adaptive Immunity

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Immunological Memory and Tolerance

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