Principal goal of hydroxyurea therapy is to:

Correct Answer: D

Rationale: The principal goal of hydroxyurea therapy is to increase circulating fetal hemoglobin (HbF), which has anti-sickling properties and reduces the frequency of sickle cell crises. Hydroxyurea stimulates HbF production by inducing the expression of gamma-globin genes. This helps to inhibit the polymerization of sickle hemoglobin (HbS) and decrease the proportion of sickled cells in circulation. Choices A and B are incorrect because the goal is not to decrease circulating HbS or increase its polymerization. Choice C is incorrect as hydroxyurea does not directly affect the water content of red cells. The correct answer, choice D, aligns with the therapeutic mechanism of hydroxyurea in increasing levels of HbF to improve clinical outcomes in sickle cell disease.

Correct Answer: A

Rationale: The correct answer is A: Chronic atrophic gastritis. This condition leads to vitamin B12 deficiency, causing macrocytic anemia, weakness, and tingling. Chronic atrophic gastritis affects the stomach's ability to produce intrinsic factor needed for B12 absorption. Ileal resection impairs B12 absorption, tapeworm infestation competes for B12, and alcoholism can lead to poor diet and malabsorption, all contributing to macrocytic anemia.

Correct Answer: A

Rationale: The correct answer is A because t(c-myc) in Burkitt's lymphoma is actually a translocation between chromosome 8 and 14 or less commonly 8 and 22. This translocation leads to the overexpression of c-myc oncogene. Choice B is correct as Burkitt's lymphoma often presents with abdominal involvement in HIV patients. Choice C is true as the characteristic "starry sky" appearance is due to the high number of macrophages engulfing apoptotic lymphoma cells. Choice D is also true, as cyclophosphamide is a common chemotherapy drug used to treat Burkitt's lymphoma.

Correct Answer: B

Rationale: The correct answer is B: Reticulocytes. Reticulocytes are immature red blood cells released by the bone marrow into the bloodstream during erythropoiesis. They still contain remnants of ribosomal RNA, giving them a reticulated appearance under a microscope. Myelocytes (A) are immature white blood cells, not involved in erythropoiesis. Ring sideroblasts (C) are found in the bone marrow in certain types of anemia, not a direct sign of erythropoiesis. Target cells (D) are red blood cells with a target-like appearance due to excess membrane, not specific to erythropoiesis.

Correct Answer: C

Rationale: The correct answer is C: Binding antithrombin III. Heparin enhances the activity of antithrombin III, which in turn inhibits several clotting factors, specifically thrombin and factor Xa. This prevents the formation of fibrin clots. Choices A, B, and D are incorrect because heparin does not inhibit vitamin K factors, prevent platelet activation, or activate tissue plasminogen activator (tPA) directly. Heparin works primarily by enhancing the inhibitory action of antithrombin III on clotting factors.