The lipids of the RBC membrane are arranged:

Correct Answer: D

Rationale: The correct answer is D because the lipids in the RBC membrane are arranged in a phospholipid bilayer structure, with two layers that are not symmetric in composition. This asymmetry is crucial for maintaining the integrity and function of the membrane. The inner and outer layers have different lipid compositions, allowing for specific functions such as signal transduction and selective permeability. Choice A is incorrect because lipids are not arranged in chains beneath a protein exoskeleton in the RBC membrane. Choice B is incorrect because the hydrophobic portions of lipids face each other within the membrane, not the plasma. Choice C is incorrect as lipids are not arranged in a hexagonal lattice in the RBC membrane.

Correct Answer: B

Rationale: The correct answer is B (25%). When both parents are carriers of the sickle cell trait, they each pass down the gene for sickle cell or normal hemoglobin with a 50% chance. The possible combinations are SS (normal), SS (sickle cell disease), AS (sickle cell trait), or AS (sickle cell trait). Therefore, there is a 25% chance (1 out of 4) that their child will inherit two sickle cell genes and have sickle cell anaemia. Choices A, C, and D are incorrect as they do not accurately reflect the genetic probability based on Mendelian inheritance principles.

Correct Answer: B

Rationale: The most likely diagnosis is B: Deep vein thrombosis (DVT). In a post-hip replacement patient with painful, pale, and swollen calf, DVT is common due to immobility and hypercoagulability. DVT presents with unilateral swelling, pain, and discoloration. Acute limb ischemia (A) typically presents with sudden severe pain, pallor, pulselessness, and paralysis. Cellulitis (C) presents with redness, warmth, and tenderness, not pallor. Fracture complication (D) would typically present with localized pain and swelling at the fracture site, not in the calf region.

Correct Answer: A

Rationale: The correct answer is A: potentially lethal graft-versus-host disease. Bone marrow transplantation in immunocompromised patients can lead to graft-versus-host disease, where the donor's immune cells attack the recipient's tissues, potentially leading to severe complications or death. This occurs due to the recognition of the recipient's tissues as foreign by the donor's immune system. The other choices are incorrect because B (high risk of T cell leukemia) is not a major problem associated with bone marrow transplantation, C (inability to use a live donor) is not a major problem but a limitation, and D (delayed hypersensitivity) is not a major concern in this context.

Correct Answer: C

Rationale: Neutrophils are attracted to an infected area by phagocytosis of IgE-coated bacteria. This is because IgE antibodies bind to pathogens, activating neutrophils through Fc receptors for phagocytosis. IgM (A) is not directly involved in neutrophil attraction. Vascular permeability (B) is a response to inflammation but does not directly attract neutrophils. Aggregation of C4 and C2 (D) is part of the complement system and does not specifically attract neutrophils.