What causes intestinal symptoms?

Correct Answer: C

Rationale: Intestinal symptoms like diarrhea, vomiting, and abdominal pain stem from toxins disrupting gut function. Enterotoxins, produced by bacteria such as Escherichia coli or Staphylococcus aureus, target intestinal cells, increasing secretion or reducing absorption by altering ion channels or tight junctions, as seen in cholera. Cytotoxins cause broader cell damage, not specific to the gut. Neurotoxins affect nerves, not digestion directly. Leukocidins attack immune cells, not intestinal ones. Enterotoxins' specific action on gut epithelium, leading to fluid loss and characteristic symptoms, distinguishes them as the cause, critical for diagnosing gastrointestinal bacterial infections.

Correct Answer: C

Rationale: Most bacteria thrive at 37°C (98.6°F), mimicking human body temperature, ideal for pathogens like E. coli in lab cultures. An incubator maintains this stable warmth, promoting growth. A lighted window (22°C) varies and is cooler, a refrigerator (7°C) slows metabolism, and a freezer (-12°C) halts it. The incubator's optimal temperature, matching bacterial physiology, ensures rapid division and colony formation, making it the best choice for cultivating diverse species.

Correct Answer: C

Rationale: Tropomyosin moves off actin's myosin-binding sites when Ca²⁺ binds troponin, which shifts tropomyosin, allowing myosin to bind actin for contraction. Ca²⁺ and troponin don't move off they enable the shift. Troponin doesn't block sites or move myosin it's regulatory. Troponin moving tropomyosin reverses roles tropomyosin shifts due to troponin. Tropomyosin's displacement by troponin-Ca²⁺ distinguishes it, key to exposing sites, unlike static or reversed pairings, central to sliding filament theory.

Correct Answer: A

Rationale: Smooth and skeletal muscles differ structurally and functionally. Smooth muscle lines artery walls, enabling involuntary constriction to regulate blood flow, unlike skeletal muscle, which attaches to bones for voluntary movement. Skeletal muscle is multinucleate and striated, lacking intercalated discs features of cardiac muscle not smooth muscle, which is uninucleate and non-striated. Smooth muscle's presence in visceral organs, controlled by the autonomic nervous system, contrasts with skeletal muscle's somatic control, highlighting its role in automatic processes like circulation, distinct from skeletal muscle's locomotive purpose.

Correct Answer: C

Rationale: Muscle roles define movement dynamics. An antagonist opposes or reverses an agonist's action, relaxing during the agonist's contraction e.g., triceps antagonize biceps in elbow flexion. Agonists drive the primary motion, synergists assist, and fixators stabilize. The antagonist's counteraction ensures controlled, reversible movements, a key biomechanical principle distinguishing it from supportive or driving roles, essential for coordinated skeletal motion.