Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 261: Which of the following factors does NOT affect the affinity of hemoglobin for oxygen?

A. pH
B. Temperature
C. 2,3-DPG
D. PCO2 (Correct Answer)

Explanation: ### Explanation The affinity of hemoglobin (Hb) for oxygen is primarily illustrated by the **Oxygen-Dissociation Curve (ODC)**. A shift to the right indicates decreased affinity (easier unloading), while a shift to the left indicates increased affinity. **Why PCO2 is the Correct Answer:** Technically, all four factors listed influence the ODC. However, in the context of standard physiological teaching and competitive exams like NEET-PG, a distinction is often made between factors that change the **shape/position** of the curve versus those that directly alter the **chemical structure** of the Hb molecule's binding site. *Note on the Question:* In many classic physiology texts (like Guyton), PCO2, pH, and Temperature are all listed as factors. However, if forced to choose which "does not affect affinity" in a specific biochemical sense, some examiners argue that **PCO2** acts primarily through the **Bohr Effect** (by changing pH) or that the question is testing the specific molecular binding of 2,3-DPG. *However, if this is a recall question where PCO2 is marked correct, it often implies that PCO2's effect is secondary to the pH change it induces.* **Analysis of Incorrect Options:** * **A. pH:** A decrease in pH (acidosis) reduces Hb affinity for O2, shifting the curve to the right (Bohr Effect). * **B. Temperature:** Increased temperature (as seen in metabolically active tissues) weakens the bond between Hb and O2, decreasing affinity. * **C. 2,3-DPG:** This byproduct of glycolysis binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (Tense) state and significantly decreasing oxygen affinity. **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis, **D**PG increase, **E**xercise, **T**emperature increase. * **Left Shift:** Fetal Hemoglobin (HbF) has a higher affinity for O2 than adult Hb (HbA) because it does not bind 2,3-DPG effectively. * **P50:** The partial pressure of O2 at which Hb is 50% saturated. Normal value is **26.6 mmHg**. An increase in P50 signifies a decrease in affinity (Right shift).

Question 262: Arrange the following clotting factors in sequence that they follow in the coagulation cascade: 1. Factor XIII, 2. Factor XII, 3. Factor X, 4. Factor V, 5. Factor IX?

A. 2-3-4-1-5
B. 2-5-3-4-1 (Correct Answer)
C. 1-2-5-3-4
D. 2-3-5-4-1

Explanation: ### Explanation The coagulation cascade is divided into the **Intrinsic**, **Extrinsic**, and **Common pathways**. To solve this sequence, one must trace the flow from the initiation of the intrinsic pathway to the final stabilization of the fibrin clot. 1. **Factor XII (Hageman factor):** Initiates the Intrinsic pathway upon contact with collagen or negatively charged surfaces. 2. **Factor IX (Christmas factor):** Activated by Factor XIa (within the intrinsic pathway). 3. **Factor X (Stuart-Prower factor):** The starting point of the **Common Pathway**. It is activated by the "Tenase complex" (IXa + VIIIa). 4. **Factor V (Proaccelerin):** Acts as a cofactor for Factor Xa to form the **Prothrombinase complex**, which converts Prothrombin to Thrombin. 5. **Factor XIII (Fibrin Stabilizing Factor):** The final step; it cross-links fibrin monomers into a stable polymer. Thus, the correct sequence is **2-5-3-4-1**. #### Analysis of Incorrect Options: * **Option A & D:** These place Factor X (3) before Factor IX (5). In the intrinsic pathway, IX must activate X. * **Option C:** This incorrectly starts with Factor XIII (1), which is the terminal factor of the cascade. #### NEET-PG High-Yield Pearls: * **Common Pathway mnemonic:** "Factors **10, 5, 2, 1**" (X, V, II, I). * **Vitamin K Dependent Factors:** II, VII, IX, and X (and Proteins C and S). * **Lab Correlation:** The **aPTT** measures the Intrinsic and Common pathways; **PT** measures the Extrinsic (Factor VII) and Common pathways. * **Factor XIII Deficiency:** Characterized by normal PT/aPTT but poor wound healing and delayed bleeding; diagnosed via the **Urea Solubility Test**.

Question 263: Which of the following substances inhibits platelet aggregation?

A. Adenosine diphosphate (ADP)
B. Thromboxane A2
C. Thrombin
D. Bradykinin (Correct Answer)

Explanation: **Explanation:** The regulation of platelet aggregation is a balance between pro-aggregatory (pro-thrombotic) and anti-aggregatory (anti-thrombotic) factors. **Why Bradykinin is correct:** Bradykinin is a potent vasodilator and an indirect inhibitor of platelet aggregation. It acts on the vascular endothelium to stimulate the release of **Nitric Oxide (NO)** and **Prostacyclin (PGI2)**. Both NO and PGI2 increase intracellular cyclic nucleotides (cGMP and cAMP, respectively) within platelets, which prevents their activation and aggregation. This mechanism ensures that the vascular lumen remains patent and prevents unnecessary clot formation. **Analysis of Incorrect Options:** * **A. Adenosine diphosphate (ADP):** Released from platelet dense granules, ADP is a powerful agonist that binds to P2Y1 and P2Y12 receptors, promoting platelet shape change and aggregation. * **B. Thromboxane A2 (TXA2):** A product of the cyclooxygenase (COX) pathway in platelets, TXA2 is a potent vasoconstrictor and platelet aggregator. Aspirin works by irreversibly inhibiting the COX-1 enzyme, thereby reducing TXA2 levels. * **C. Thrombin:** Known as the most potent platelet activator, thrombin converts fibrinogen to fibrin and activates platelets via Protease-Activated Receptors (PARs). **High-Yield NEET-PG Pearls:** * **Endogenous Inhibitors:** Prostacyclin (PGI2), Nitric Oxide, and Ecto-ADPase (CD39) are the primary physiological inhibitors of platelet aggregation. * **Platelet Receptors:** P2Y12 is the target for drugs like **Clopidogrel** and **Ticagrelor**. * **GP IIb/IIIa:** This is the "final common pathway" for platelet aggregation; its deficiency leads to **Glanzmann Thrombasthenia**.

Question 264: What is a characteristic feature of fetal red blood cells compared to adult red blood cells?

A. Resistant to alkali denaturation (Correct Answer)
B. Smaller in size
C. Higher 2,3-DPG level
D. Contains more iron

Explanation: ### Explanation **1. Why Option A is Correct: Resistance to Alkali Denaturation** The primary hemoglobin in fetal red blood cells (RBCs) is **Hemoglobin F (HbF)**, which consists of two alpha ($\alpha$) and two gamma ($\gamma$) chains. A key biochemical property of the $\gamma$-chains is their inherent resistance to denaturation by alkaline solutions (like KOH). In contrast, adult hemoglobin (HbA) is rapidly denatured by alkali. This difference forms the basis of the **Apt test**, used clinically to distinguish fetal blood from maternal blood in cases of neonatal gastrointestinal bleeding or vaginal bleeding during pregnancy. **2. Why Other Options are Incorrect:** * **Option B (Smaller in size):** Fetal RBCs are actually **larger** than adult RBCs. They are macrocytic, with a Mean Corpuscular Volume (MCV) often exceeding 110-120 fL at birth, compared to the adult average of 80-100 fL. * **Option C (Higher 2,3-DPG level):** Fetal RBCs do not have higher 2,3-DPG levels. More importantly, **HbF has a lower affinity for 2,3-DPG** than HbA. This allows HbF to maintain a higher affinity for oxygen, facilitating the transfer of oxygen from maternal blood to the fetus across the placenta. * **Option D (Contains more iron):** While the total iron stores in a neonate are high, the individual hemoglobin molecule contains the same four iron atoms (heme groups) as adult hemoglobin. **3. High-Yield Clinical Pearls for NEET-PG:** * **P50 Value:** The P50 (partial pressure of $O_2$ at which Hb is 50% saturated) is **lower** in fetal blood (~19 mmHg) than in adult blood (~27 mmHg), reflecting a left shift in the oxygen dissociation curve. * **Lifespan:** Fetal RBCs have a shorter lifespan (**80–90 days**) compared to adult RBCs (120 days). * **HbF Structure:** $\alpha_2\gamma_2$. The switch from $\gamma$ to $\beta$ chain synthesis begins before birth but is completed around 6 months of age.

Question 265: What is the first step in the initiation of primary hemostasis for clot formation?

A. Fibrin deposition
B. Vasoconstriction (Correct Answer)
C. Platelet adhesion
D. Thrombosis

Explanation: **Explanation:** Hemostasis is the physiological process that stops bleeding at the site of vascular injury. It occurs in three overlapping stages: **Vascular spasm (Vasoconstriction)**, **Primary Hemostasis** (Platelet plug formation), and **Secondary Hemostasis** (Coagulation cascade). **Why Vasoconstriction is the correct answer:** Immediately upon vascular injury, the smooth muscle in the vessel wall contracts. This is the **first and immediate response**. It is triggered by direct myogenic spasm, local autacoid factors (like Endothelin-1 released from damaged endothelium), and nervous system reflexes. The primary goal is to reduce blood flow to the injured area, minimizing blood loss and allowing pro-coagulant factors and platelets to accumulate at the site. **Analysis of Incorrect Options:** * **Platelet Adhesion:** This is the second step of primary hemostasis. It occurs only after vasoconstriction, when platelets bind to exposed subendothelial collagen via von Willebrand Factor (vWF). * **Fibrin Deposition:** This is the end-product of **Secondary Hemostasis** (the coagulation cascade). It stabilizes the initial platelet plug into a definitive clot. * **Thrombosis:** This is a pathological state where a clot forms within an intact blood vessel, rather than a physiological step in hemostasis. **NEET-PG High-Yield Pearls:** * **Sequence of Primary Hemostasis:** Vasoconstriction → Platelet Adhesion (GpIb-vWF) → Platelet Activation/Degranulation (ADP, TXA2) → Platelet Aggregation (GpIIb/IIIa). * **Potent Vasoconstrictor:** Endothelin-1 is the most potent endogenous vasoconstrictor released during vascular injury. * **Platelet Plug vs. Clot:** Primary hemostasis results in a "white thrombus" (unstable), while secondary hemostasis results in a "red thrombus" (stable fibrin mesh).

Question 266: Lysozyme is present in which of the following?

A. Saliva (Correct Answer)
B. Human milk
C. Tears
D. Mucus

Explanation: **Explanation:** Lysozyme (also known as muramidase) is a critical enzyme of the **innate immune system**. It functions by hydrolyzing the β-1,4 glycosidic bonds between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in the peptidoglycan layer of bacterial cell walls, primarily targeting Gram-positive bacteria. While the question identifies **Saliva (Option A)** as the correct answer, it is important to note that this is a "single best response" scenario. Lysozyme is a ubiquitous enzyme found in almost all body secretions. * **Saliva (A):** It contains lysozyme, secretory IgA, and lactoferrin, which together provide the first line of defense against oral pathogens. * **Human Milk (B):** Lysozyme is present in high concentrations in breast milk (especially colostrum) and remains stable in the infant's gut to provide passive immunity. * **Tears (C):** The lacrimal glands secrete high amounts of lysozyme to protect the cornea and conjunctiva from infection. * **Mucus (D):** Respiratory and gastrointestinal mucus contain lysozyme to trap and degrade inhaled or ingested pathogens. **Note on Question Quality:** In many competitive exams like NEET-PG, if a question asks "where is it present" and all options are technically correct, the examiner may be looking for the most "classic" or "abundant" source, or the question may be flawed. However, historically, saliva and tears are the most frequently tested sources. **Clinical Pearls for NEET-PG:** 1. **Source:** Lysozyme is primarily synthesized by **neutrophils, monocytes, and macrophages**. 2. **Marker:** Elevated serum lysozyme levels are a diagnostic marker for **Sarcoidosis** and certain leukemias (specifically **AML-M4 and M5**). 3. **Paneth Cells:** In the small intestine, Paneth cells secrete large amounts of lysozyme to maintain mucosal immunity.

Question 267: In fetal life, which organs are primarily responsible for the production of red blood cells?

A. Prenatal life (Correct Answer)
B. First year of life
C. Up to five years of life
D. Lifelong

Explanation: ### Explanation The production of red blood cells (erythropoiesis) undergoes a series of transitions in location during human development. The question focuses on the primary site of production during **fetal life**, which corresponds to the **prenatal period**. **1. Why "Prenatal life" is correct:** During fetal development, erythropoiesis occurs in three distinct stages: * **Mesoblastic Stage (Weeks 3–8):** Occurs in the **yolk sac**. * **Hepatic Stage (Month 2 – Birth):** The **liver** becomes the primary site, with the spleen also contributing. * **Myeloid Stage (Month 5 onwards):** The **bone marrow** begins production and becomes the dominant site by the time of birth. Since all these stages occur before birth, "Prenatal life" is the accurate timeframe for these specific organ involvements. **2. Why the other options are incorrect:** * **First year of life & Up to five years:** After birth, the liver and spleen cease erythropoiesis. The **red bone marrow** of virtually all bones (long and flat) takes over. * **Lifelong:** While erythropoiesis is lifelong, the *organs* responsible change. In adults (after age 20), production is restricted to the **membranous/flat bones** (vertebrae, sternum, ribs, ilia) as the shaft of long bones undergoes fatty replacement (yellow marrow). **3. High-Yield Clinical Pearls for NEET-PG:** * **Order of sites:** Yolk sac → Liver (dominant) → Spleen → Bone Marrow. * **Extramedullary Erythropoiesis:** In certain pathological states (e.g., Thalassemia, Myelofibrosis), the liver and spleen can resume RBC production in adults. * **Hormonal Control:** Fetal erythropoiesis is initially independent of erythropoietin but later becomes dependent on it (produced primarily by the fetal liver, then the kidneys).

Question 268: Which of the following cells become tissue macrophages?

A. Neutrophils
B. Eosinophils
C. Monocytes (Correct Answer)
D. Lymphocytes

Explanation: **Explanation:** **Monocytes** are the largest type of white blood cells and are considered immature cells while circulating in the bloodstream. They typically circulate for about 10–20 hours before migrating through the capillary walls into various tissues. Once they enter the tissues, they undergo significant enlargement and structural changes (increasing their lysosomal content and phagocytic capacity) to become **Tissue Macrophages**. This transition is part of the **Mononuclear Phagocyte System** (formerly known as the Reticuloendothelial System). **Why the other options are incorrect:** * **Neutrophils (A):** These are "first responders" to acute inflammation. While they are highly phagocytic, they are short-lived "professional" killers that die after phagocytosis (forming pus) and do not transform into macrophages. * **Eosinophils (B):** These are primarily involved in combating parasitic infections and mediating allergic responses. They do not differentiate into macrophages. * **Lymphocytes (D):** These are the primary cells of the adaptive immune system (B-cells and T-cells). They differentiate into plasma cells or effector T-cells, not macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Tissue-Specific Names:** Macrophages have specific names depending on their location: * **Liver:** Kupffer cells * **Lungs:** Alveolar macrophages (Dust cells) * **CNS:** Microglia * **Skin:** Langerhans cells * **Bone:** Osteoclasts * **Kidney:** Mesangial cells * **Function:** Macrophages act as **Antigen-Presenting Cells (APCs)**, linking the innate and adaptive immune systems by presenting processed antigens to T-lymphocytes via MHC II molecules.

Question 269: A baby's blood group is determined as O Rh negative. Which of the following blood groups would neither the mother nor the father have?

A. A, Rh Positive
B. B, Rh Positive
C. AB, Rh Negative (Correct Answer)
D. O, Rh Positive

Explanation: ### Explanation **1. Underlying Medical Concept** Blood group inheritance follows **Mendelian genetics**. The ABO system is governed by three alleles: $I^A$, $I^B$ (codominant), and $i$ (recessive). * A person with **Blood Group O** must have the genotype **$ii$**. This means they must inherit one '$i$' allele from each parent. * A person with **Blood Group AB** has the genotype **$I^AI^B$**. They can only pass on an '$I^A$' or an '$I^B$' allele to their offspring. Therefore, a parent with blood group AB **cannot** have a biological child with blood group O, as they cannot provide the necessary '$i$' allele. Regarding the **Rh system**, Rh-negative is a recessive trait ($dd$). An Rh-negative child ($dd$) can be born to Rh-positive parents if both parents are heterozygous ($Dd$). However, the ABO incompatibility in this question is the absolute "rule-out" factor. **2. Analysis of Options** * **Option C (AB, Rh Negative): Correct.** As explained, an AB parent lacks the '$i$' allele required to produce an O-group child. * **Option A & B (A or B, Rh Positive): Incorrect.** Parents with phenotype A or B can be heterozygous (genotypes $Ai$ or $Bi$). If both parents carry the '$i$' allele, they can produce an O ($ii$) child. If they are also heterozygous for Rh ($Dd$), they can produce an Rh-negative ($dd$) child. * **Option D (O, Rh Positive): Incorrect.** An O-group parent ($ii$) is the most likely source of the '$i$' allele for an O-group child. **3. Clinical Pearls for NEET-PG** * **Bombay Blood Group:** Phenotypically appears as 'O' but lacks the H-antigen. They can only receive blood from another Bombay phenotype individual. * **Erythroblastosis Fetalis:** Occurs when an Rh-negative mother carries an Rh-positive fetus. The first pregnancy is usually safe; subsequent pregnancies are at risk unless **Anti-D (RhoGAM)** is administered at 28 weeks and within 72 hours of delivery. * **Universal Donor:** O Negative (packed RBCs); **Universal Recipient:** AB Positive.

Question 270: The Schilling test is used to diagnose which condition?

A. Vitamin B12 deficiency (Correct Answer)
B. Folic acid deficiency
C. Vitamin B6 deficiency
D. Vitamin D deficiency

Explanation: **Explanation:** The **Schilling test** is a classic diagnostic tool used to evaluate the absorption of **Vitamin B12 (Cobalamin)**. It is specifically designed to differentiate between various causes of B12 deficiency, such as Pernicious Anemia (lack of Intrinsic Factor), malabsorption syndromes, or bacterial overgrowth. **Why Vitamin B12 is correct:** The test involves administering an oral dose of radiolabeled Vitamin B12 followed by an intramuscular "flushing dose" of unlabeled B12. If the radiolabeled B12 is absorbed in the terminal ileum, it is excreted in the urine. Low urinary excretion suggests malabsorption. By repeating the test with the addition of **Intrinsic Factor (IF)**, clinicians can confirm **Pernicious Anemia** if the absorption normalizes. **Why other options are incorrect:** * **Folic acid deficiency:** While clinically similar to B12 deficiency (both cause megaloblastic anemia), folate absorption does not require Intrinsic Factor and is not assessed by the Schilling test. * **Vitamin B6 deficiency:** This typically leads to microcytic hypochromic anemia (sideroblastic anemia) and is diagnosed via serum levels or functional assays, not absorption tests. * **Vitamin D deficiency:** This is assessed by measuring serum 25-hydroxyvitamin D levels and relates to calcium metabolism and bone health. **NEET-PG Clinical Pearls:** * **Site of Absorption:** Vitamin B12 is absorbed in the **terminal ileum**; hence, Crohn’s disease or ileal resection can lead to a positive Schilling test. * **Pernicious Anemia:** This is an autoimmune destruction of gastric parietal cells, leading to a deficiency of Intrinsic Factor. * **Modern Practice:** Though high-yield for exams, the Schilling test is now largely replaced by anti-intrinsic factor antibodies and serum methylmalonic acid (MMA) levels. * **Classic Triad:** Megaloblastic anemia + Neurological symptoms (Subacute Combined Degeneration of Spinal Cord) + Glossitis = Vitamin B12 deficiency.

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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