Which one of the following does NOT contain C3b?

Correct Answer: C

Rationale: The correct answer is C because classic-pathway C3 convertase does not contain C3b. In the classical pathway, C3 convertase is formed by the cleavage of C4bC2a, which does not involve C3b. A: classic-pathway C5 convertase contains C3b as it is formed by the binding of C4b2a3b complex to C5. B: alternative-pathway C5 convertase contains C3b as it is formed by the binding of C3bBb3b complex to C5. D: alternative-pathway C3 convertase contains C3b as it is formed by the binding of C3bBb complex to C3.

Correct Answer: A

Rationale: The correct answer is A because allelic exclusion in this context refers to the phenomenon where only one allelic form of a gene is expressed in a particular cell. In this case, the individual is heterozygous for Gm allotypes, meaning they have two different allelic forms of IgG. Since the individual lymphocytes produce only one of the two forms, it suggests that the heavy-chain gene on one chromosome has undergone rearrangement, leading to the expression of a single form of IgG. This supports choice A. Choices B, C, and D are incorrect because they involve either both chromosomes or light-chain genes, which are not relevant to the scenario described.

Correct Answer: B

Rationale: The correct answer is B: a disturbance in homeostasis. When a human system fails to function properly, homeostasis, the body's ability to maintain internal stability, is disrupted. This can lead to various health issues as the body struggles to maintain balance. Choices A, C, and D do not directly address the concept of homeostasis or the consequences of system failure, making them incorrect. Choice A (a stable rate of metabolism) does not reflect the impact of system failure on homeostasis. Choice C (a change in the method of cellular respiration) is too specific and does not encompass the broader consequences of system failure. Choice D (a change in the function of DNA) is not directly related to the immediate effects of system failure on the body's internal stability.

Correct Answer: B

Rationale: The correct answer is B because pneumocystis jiroveci pneumonia (PCP) results from an impaired immune system. The organism causing PCP exists as part of the normal flora of the lungs but becomes an aggressive pathogen when the immune system is compromised. This is why PCP is commonly seen in individuals with HIV/AIDS or other conditions that weaken the immune system. Choice A is incorrect because PCP is not contracted from pets or through toxoplasmosis transmitted via cat feces. Choice C is incorrect because PCP is not spread through airborne droplets but rather through the inhalation of the fungus. Choice D is incorrect because PCP does not require direct blood contact for transmission; it is primarily contracted through the inhalation of the fungus.

Correct Answer: B

Rationale: Correct Answer: B Rationale: 1. Systemic scleroderma is a connective tissue disease characterized by skin tightening, affecting blood vessels and internal organs. 2. Skin tightening in systemic scleroderma can lead to contractures if not managed with exercises. 3. Nurse must expect skin changes and educate on the importance of range-of-motion and muscle-strengthening exercises. 4. Increased joint flexibility (Choice C) is not expected in systemic scleroderma due to skin and tissue tightening. 5. Excessive salivation (Choice A) and frequent nosebleeds (Choice D) are not typical manifestations of systemic scleroderma.