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Which of the following enzymes is directly involved in the conversion of ribonucleotides to deoxyribonucleotides, crucial for DNA synthesis in rapidly dividing hematopoietic cells?
A. Thymidylate synthase
B. DNA polymerase
C. Ribonucleotide reductase
D. Adenosine deaminase
✅ Answer: C. Ribonucleotide reductase
Explanation: Ribonucleotide reductase catalyzes the conversion of ribonucleotides (e.g., ADP, GDP) to deoxyribonucleotides, essential for DNA synthesis. This is critical in proliferating cells, including bone marrow cells.
What is the primary stimulus for erythropoietin (EPO) secretion?
A. Hypercapnia
B. Hypoxia
C. Low serum ferritin
D. Elevated 2,3-BPG levels
✅ Answer: B. Hypoxia
Explanation: Hypoxia (low oxygen tension) in the kidney stimulates peritubular interstitial cells to release erythropoietin, which acts on the bone marrow to increase red blood cell production.
The bone marrow responsible for hematopoiesis in adults is predominantly located in which of the following sites?
A. Diaphysis of long bones
B. Epiphysis of femur
C. Flat bones such as the sternum and pelvis
D. Cortical bone of ribs
✅ Answer: C. Flat bones such as the sternum and pelvis
Explanation: In adults, active (red) marrow is found primarily in flat bones (sternum, pelvis, ribs) and the proximal ends of long bones.
Histology
The spleen plays an important role in the hematologic system. Which part of the spleen is primarily involved in the destruction of senescent red blood cells?
A. White pulp
B. Marginal zone
C. Red pulp
D. Splenic capsule
✅ Answer: C. Red pulp
Explanation: The red pulp of the spleen filters the blood and removes old or damaged red blood cells via macrophages in the splenic cords.
Which of the following best describes the appearance of megakaryocytes under a microscope?
A. Small round cells with basophilic cytoplasm
B. Large multinucleated cells with abundant cytoplasm
C. Small cells with a clear perinuclear halo
D. Biconcave discs without nuclei
✅ Answer: B. Large multinucleated cells with abundant cytoplasm
Explanation: Megakaryocytes are large cells in the bone marrow that produce platelets. They have a lobulated nucleus and abundant granular cytoplasm.
Pharmacology
Which of the following drugs acts by irreversibly inhibiting cyclooxygenase in platelets?
A. Clopidogrel
B. Heparin
C. Aspirin
D. Warfarin
✅ Answer: C. Aspirin
Explanation: Aspirin irreversibly inhibits COX-1 and COX-2, preventing the formation of thromboxane A2 in platelets, thus inhibiting aggregation.
Which of the following anticoagulants acts by enhancing the activity of antithrombin III?
A. Warfarin
B. Dabigatran
C. Heparin
D. Rivaroxaban
✅ Answer: C. Heparin
Explanation: Heparin potentiates the activity of antithrombin III, which inactivates thrombin and factor Xa, leading to anticoagulation.
Which of the following is responsible for the chloride shift (Hamburger phenomenon) in red blood cells?
A. Carbonic anhydrase
B. Hemoglobin saturation
C. Na+/K+ ATPase
D. Band 3 protein
✅ Answer: D. Band 3 protein
Explanation: Band 3 is an anion exchanger in RBCs that facilitates the exchange of bicarbonate and chloride ions across the membrane, important in CO₂ transport.
A deficiency in which of the following enzymes results in hemolytic anemia due to impaired protection against oxidative damage?
A. Pyruvate kinase
B. Glucose-6-phosphate dehydrogenase
C. Hexokinase
D. Aldolase B
✅ Answer: B. Glucose-6-phosphate dehydrogenase
Explanation: G6PD is crucial for producing NADPH, which protects RBCs from oxidative stress. Its deficiency leads to hemolytic anemia under oxidative conditions.
Which hematopoietic lineage gives rise to platelets?
A. Myeloblast
B. Monoblast
C. Megakaryoblast
D. Erythroblast
✅ Answer: C. Megakaryoblast
Explanation: Megakaryoblasts mature into megakaryocytes, which then release platelets (thrombocytes) into circulation.
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