Principal biochemical defect in sickle hemoglobin involves which substitution at the sixth position of the beta chain?

Correct Answer: B

Rationale: The correct answer is B: Valine for glutamic acid. This substitution occurs in sickle hemoglobin, where a single nucleotide change leads to the replacement of glutamic acid by valine at the sixth position of the beta chain. This change results in the characteristic sickling of red blood cells. Choices A, C, and D do not correspond to the specific substitution seen in sickle hemoglobin and are therefore incorrect. It is crucial to understand the molecular basis of the disease to differentiate the correct answer from the distractors.

Correct Answer: C

Rationale: The correct answer is C, Aplastic anemia. Aplastic anemia is characterized by pancytopenia, which includes low red blood cell count (MCV 90 fL) and low reticulocyte production index. Thalassemia (A) would typically present with microcytic anemia, not normocytic. Vitamin B12 deficiency (B) would present with macrocytic anemia. Iron deficiency anemia (D) would present with microcytic anemia. Therefore, based on the given data, the most likely cause is aplastic anemia.

Correct Answer: C

Rationale: The correct answer is C: Lymphocyte predominance. Reed-Sternberg cells are characteristic of Hodgkin's lymphoma. In this type, Reed-Sternberg cells are infrequent, and popcorn cells are present. This presentation is typical of lymphocyte predominance subtype. Nodular sclerosis (A) is characterized by bands of collagen separating nodules, mixed cellularity (B) has a mixed cell population with frequent Reed-Sternberg cells, and lymphocyte depleted (D) has few lymphocytes and more Reed-Sternberg cells.

Correct Answer: D

Rationale: The correct answer is D because iron is better absorbed in the ferrous form compared to the ferric form. Ferrous iron is more soluble and readily absorbed by the body through the duodenum and upper small intestine. Intrinsic factor is needed for the absorption of vitamin B12, not iron (Choice B). Ascorbic acid actually enhances iron absorption by reducing ferric iron to the more absorbable ferrous form, so Choice C is incorrect. Large intestine absorption (Choice A) is not a significant route for iron absorption in the body.

Correct Answer: C

Rationale: Platelets play a crucial role in the process of hemostasis after vascular injury. When a blood vessel is damaged, platelets adhere to the site and change shape to form a plug, a process known as platelet aggregation. This is important for stopping bleeding. Vasodilation (choice A) typically occurs after vasoconstriction to help with blood flow. Clotting factors (choice B) are essential in the coagulation cascade but not directly involved in platelet aggregation. Serotonin (choice D) is released by platelets during aggregation but does not directly stimulate the process. Therefore, the correct answer is C as platelets changing shape is a key step in platelet aggregation during vascular injury.