What is the diagnosis? (35-year-old with blurred vision, headaches, SOB, splenomegaly, JAK-2 mutation)

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: 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.

Correct Answer: D

Rationale: The correct answer is D because when adult AB spleen cells are injected into a newborn B mouse, the recipient's immune system is still developing. This allows the injected cells to establish tolerance towards strain A antigens. This process is known as neonatal tolerance induction. Option A is incorrect because the immune system of the recipient is not mature enough to mount a strong response to destroy the injected cells. Option B is unlikely as the injected cells would likely interact with the recipient's immune system. Option C is less likely as the immune system of the newborn mouse is not fully developed to initiate a graft-versus-host reaction efficiently.

Correct Answer: B

Rationale: The correct answer is B because cytotoxic T cells recognize and kill target cells that are infected by the specific virus that induced their activation. This is achieved through the recognition of viral antigens presented on class I MHC molecules. Therefore, the target cells must be infected by virus A and have antigens presented on class I MHC molecules that are identical to the T cells' receptors. Choice A is incorrect because cytotoxic T cells are specific to the virus that induced their activation, not any virus from the same host. Choice C is incorrect because cytotoxic T cells primarily recognize antigens presented on class I MHC, not class II MHC. Choice D is incorrect because the target cells should be infected with the same virus that activated the cytotoxic T cells, not a different virus.