Heparin is an anticoagulant produced by the basophils. The most likely mechanism heparin prevents blood clotting is by

Correct Answer: D

Rationale: The correct answer is D because heparin binds to antithrombin III, enhancing its ability to inactivate thrombin. This prevents the conversion of fibrinogen to fibrin, thereby inhibiting blood clot formation. Choice A is incorrect because vitamin K-dependent clotting factors are not directly affected by heparin. Choice B is incorrect because heparin does not prevent the activation of platelets, rather it affects the coagulation cascade. Choice C is incorrect as heparin does not directly prevent platelet aggregation, which is more related to antiplatelet agents like aspirin.

Correct Answer: C

Rationale: The correct answer is C: pernicious anemia. Pernicious anemia is a macrocytic anemia caused by vitamin B12 deficiency, leading to large red blood cells. Sideroblastic anemia (A) is a type of microcytic anemia due to ineffective iron utilization. Thalassemia (B) causes microcytic anemia due to decreased production of hemoglobin. Lead poisoning (D) can also lead to microcytic anemia by disrupting heme synthesis. Therefore, pernicious anemia is the only choice that does not produce microcytic anemia.

Correct Answer: C

Rationale: Step-by-step rationale: 1. Hypersplenism is characterized by splenomegaly (A), causing sequestration and destruction of blood cells. 2. Pancytopenia (B) results from excessive destruction of blood cells in the enlarged spleen. 3. Hypercellular bone marrow (C) is not directly associated with hypersplenism; it is a compensatory response to increased blood cell destruction. 4. Splenectomy (D) can reverse the effects of hypersplenism by removing the source of blood cell destruction. Therefore, the correct answer is C as hypercellular bone marrow is a secondary response to hypersplenism, not a direct association.

Correct Answer: B

Rationale: The correct answer is B: gum bleeding. In immune thrombocytopenic purpura (ITP), the hallmark feature is low platelet count leading to easy bruising and bleeding, including gum bleeding. Fever (choice A) is not a typical feature of ITP. Moderate splenomegaly (choice C) is less common in ITP. Stema tenderness (choice D) is not a recognized feature of ITP. Therefore, gum bleeding is the outstanding feature of ITP due to the primary underlying mechanism of platelet destruction.

Correct Answer: C

Rationale: The correct answer is C: optic neuritis. Busulfan therapy is known to cause adverse effects such as hyperpigmentation, pulmonary fibrosis, and bone marrow suppression due to its cytotoxic effects. However, optic neuritis is not a reported side effect of busulfan therapy. Optic neuritis is more commonly associated with other medications or conditions affecting the optic nerve. Therefore, the absence of optic neuritis as a known side effect of busulfan therapy makes choice C the correct answer.