What is the distribution of normal Hb in adults?

Correct Answer: B

Rationale: The correct answer is B (>95% Hb A, <3.5% Hb A2, <1-2% Hb F). In adults, the normal distribution of hemoglobin includes mainly Hb A (>95%), a small percentage of Hb A2 (<3.5%), and an even smaller percentage of Hb F (<1-2%). This distribution is typical in healthy individuals and represents the normal composition of adult hemoglobin. Choice A (80-90% Hb A, 5-10% Hb A2, 1-5% Hb F) and Choice C (90-95% Hb A, 3-5% Hb A2, 1-3% Hb F) are incorrect because they do not reflect the accurate distribution percentages of Hb A, Hb A2, and Hb F in adults. Choice D (85-90% Hb A, 5-7% H

Correct Answer: B

Rationale: The correct diagnosis is B: Polycythemia vera. The presence of JAK-2 mutation is a key indicator for polycythemia vera. This condition is characterized by increased red blood cell production, leading to symptoms such as blurred vision, headaches, and splenomegaly. The other choices can be ruled out based on the specific clinical presentation and genetic mutation. Myelofibrosis typically presents with bone marrow fibrosis, not splenomegaly. Essential thrombocythemia is characterized by elevated platelet counts, not red blood cells. Chronic myeloid leukemia typically presents with elevated white blood cell counts and the presence of the Philadelphia chromosome, not JAK-2 mutation.

Correct Answer: C

Rationale: The correct answer is C because genetically identical individuals have the same major histocompatibility complex (MHC) antigens, so there is no immune response or rejection of grafts. Minor histocompatibility antigens (choice A) are not relevant in this case. Hyperacute rejection (choice B) occurs due to pre-existing antibodies, which is not a factor in genetically identical twins. Choice D is incorrect as both kidney and skin grafts would be accepted between identical twins due to genetic similarity.

Correct Answer: C

Rationale: The correct answer is C: antigen-IgG complexes. The classic complement pathway is initiated when C1 binds to antigen-IgG complexes. This interaction triggers a cascade of enzymatic reactions leading to opsonization, inflammation, and cell lysis. The other choices are incorrect because A (antigen) does not directly activate the complement pathway, B (factor B) is part of the alternative pathway, and D (bacterial lipopolysaccharides) activate the lectin pathway, not the classic pathway.

Correct Answer: B

Rationale: The correct answer is B because membrane-bound IgM and IgD on a B cell have identical variable regions (VH and VL) but different constant regions (CH). This is important for B cell receptor diversity while maintaining specificity. Choice A is incorrect as they have different heavy and light chains. Choice C is incorrect because they have different constant regions. Choice D is incorrect as they have identical variable regions.