Which single feature of normal RBCs is most responsible for limiting their life span?

Correct Answer: D

Rationale: The correct answer is D: Loss of nucleus. The absence of a nucleus in red blood cells (RBCs) limits their ability to repair and regenerate, leading to a shorter lifespan. Without a nucleus, RBCs cannot synthesize new proteins or repair damaged DNA, making them more susceptible to wear and tear. The other choices are incorrect because: A: Loss of mitochondria - RBCs naturally lack mitochondria to make space for more hemoglobin, but this feature does not significantly impact their lifespan. B: Increased flexibility of the cell membrane - While flexibility is important for RBCs to navigate through narrow capillaries, it does not directly affect their lifespan. C: Reduction of Hb iron - While iron is crucial for hemoglobin function, a reduction in Hb iron does not directly limit the lifespan of RBCs.

Correct Answer: B

Rationale: The correct answer is B: Fever. In diagnosing malaria, fever is a key symptom due to the cyclic nature of the disease. Malaria typically presents with recurrent bouts of fever, which is caused by the cycle of the parasite invading and multiplying in red blood cells. The other choices (A: Rash, C: Chest pain, D: Hair loss) are not typical signs of malaria. Rash is more commonly seen in diseases like dengue fever, chest pain could be indicative of other conditions like pneumonia or heart issues, and hair loss is not a characteristic symptom of malaria. Therefore, fever is the most characteristic and essential sign to consider when diagnosing malaria.

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.