What is the most likely diagnosis? (80-year-old man post-hip replacement with painful, pale, swollen left calf)

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: 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.

Correct Answer: A

Rationale: The idiotypic determinants are unique antigenic determinants located within the hypervariable regions of heavy and light chains. These regions are highly variable and contribute to the diversity of antibodies. The correct answer is A because idiotypic determinants are specific to the variable regions of antibodies, not the constant regions. Choices B, C, and D are incorrect because idiotypic determinants are not found in the constant regions or hinge region of antibodies. The variability in the hypervariable regions allows antibodies to recognize and bind to a wide range of antigens, making them crucial for the specificity of the immune response.

Correct Answer: C

Rationale: A delayed hypersensitivity reaction involves helper T cells and macrophages, causing inflammation and tissue damage over time. Helper T cells release cytokines, activating macrophages to mediate the immune response. This process leads to the formation of granulomas, which are characteristic of delayed hypersensitivity reactions. Choices A, B, and D do not align with the pathophysiology of delayed hypersensitivity reactions and are incorrect. Edema without a cellular infiltrate (Choice A) is not specific to delayed hypersensitivity reactions. An infiltrate composed of neutrophils (Choice B) is indicative of acute inflammation, not delayed hypersensitivity. Eosinophils (Choice D) are typically involved in allergic reactions and parasitic infections, not delayed hypersensitivity reactions.

Correct Answer: A

Rationale: The correct answer is A: IgG. IgG is the class of immunoglobulins that crosses the placenta from the mother to the fetus, leading to high concentrations in newborns. IgM is the first class produced in response to infection but is not present in high levels in newborns. IgA is primarily found in mucosal areas and secretions, not in high concentrations in the blood. IgD is mainly present on the surface of B cells and has limited presence in the bloodstream. Therefore, IgG is the most abundant immunoglobulin in the blood of a human newborn due to passive transfer from the mother.