Drugs like broad-spectrum antibiotics can lead to alterations in the normal microbial flora permitting the overgrowth of opportunistic organisms known as

Correct Answer: A

Rationale: Broad-spectrum antibiotics kill a wide range of bacteria, disrupting normal flora and allowing opportunistic pathogens (e.g., Clostridium difficile) to overgrow, causing superinfection. 'Super infection,' is correct, as per infectious disease texts (e.g., Mandell's Principles). 'Hypersensitivity,' is an allergic reaction, not flora-related. 'Direct toxicity,' is organ damage from the drug, not microbial imbalance. 'Toxicity,' is too broad, missing the specific overgrowth context. Superinfection's link to antibiotic-induced dysbiosis, like pseudomembranous colitis, makes A the accurate answer, distinct from immune or toxic effects.

Correct Answer: D

Rationale: Sample size is the number of units chosen to represent a population. 'Sample size,' is correct, per statistical design (e.g., Kish), determining study precision (e.g., 100 nurses). 'Sample frame,' is the population list. 'Sampling unit,' is a single entity. 'Sampling fraction,' is the proportion sampled. Sample size's focus on the count of representatives distinguishes D as the accurate answer.

Correct Answer: B

Rationale: The shoulder's ball-and-socket joint allows humeral rotation. 'Ball and socket,' is correct, the glenohumeral joint's structure, per anatomy (e.g., Tortora). 'Hinge,' (e.g., elbow) limits rotation. 'Condylar,' (e.g., knee) is less mobile. 'Saddle,' (e.g., thumb) is specific. Ball-and-socket's multidirectional range makes B the precise answer.

Correct Answer: A

Rationale: Valves in blood vessels prevent backflow, aiding venous return against gravity, but capillaries lack them due to their role in exchange, not directional flow. 'Capillaries,' is correct, as per anatomy texts like Moore's Clinically Oriented Anatomy, which note valves in veins and some venules. 'Veins,' have valves, especially in limbs. 'Venules,' may have valves in larger ones transitioning to veins. 'Vena cavae,' lack valves, but the question's focus on typical presence points to capillaries' exception. Capillaries' thin walls and continuous flow for nutrient exchange exclude valves, making A the accurate answer, distinct from the venous system's structural adaptations.

Correct Answer: B

Rationale: Sperm, the male reproductive cells, are produced in the seminiferous tubules of the testes. 'Seminiferous tubules,' is correct, per reproductive physiology texts like Tortora, where spermatogenesis occurs. 'Epididymis,' stores and matures sperm. 'Vas deferens,' transports sperm. 'Prostate gland,' produces seminal fluid. The tubules' role as the site of sperm cell formation distinguishes B as the accurate answer, foundational to male fertility.