What causes direct damage to the cells?

Correct Answer: A

Rationale: Toxins vary in their cellular targets and effects. Cytotoxins directly harm cells by disrupting membranes, inhibiting protein synthesis, or inducing apoptosis, leading to tissue damage or organ dysfunction, as seen with diphtheria toxin. Neurotoxins target nerve cells, impairing signaling, like botulinum toxin. Enterotoxins affect intestinal cells, causing symptoms like diarrhea, as with cholera toxin. Leukocidins destroy white blood cells, aiding bacterial evasion. Cytotoxins' broad cellular toxicity, applicable across cell types, contrasts with the specialized actions of others, making them the primary agents of direct cell damage in this context.

Correct Answer: B

Rationale: The small intestine maximizes nutrient absorption via villi finger-like projections lining its walls. Covered in microvilli, they vastly increase surface area, allowing efficient uptake of sugars, amino acids, and fats into the bloodstream. Venules are blood vessels, alveoli handle lung gas exchange, and nephrons filter kidney blood. Villi's specialized structure and location enhance digestion, distinguishing them as the key absorptive feature, critical for nutrient distribution in the body.

Correct Answer: B

Rationale: After nicotinic receptor activation by acetylcholine, Na⁺ ions enter, depolarizing the sarcolemma, with the action potential traveling through T-tubules to spread the signal inward. Ca²⁺ releases from the sarcoplasmic reticulum post-T-tubule signal, not depolarizing externally. K⁺ exits later to repolarize, not initiating, and sarcomeres are contractile, not conductive. Ca²⁺ in sarcomeres binds troponin, not traveling. Na⁺ via T-tubules ensures rapid, uniform activation, distinguishing it from internal Ca²⁺ or misaligned K⁺ roles, key to contraction propagation.

Correct Answer: C

Rationale: Muscle contraction follows a precise sequence. Calcium ions first enter the cytoplasm from the sarcoplasmic reticulum, triggered by a nerve impulse depolarizing the sarcolemma. This calcium binds troponin, shifting tropomyosin to expose actin's binding sites. Only then can myosin heads, energized by prior ATP hydrolysis, engage actin, pulling filaments past each other. Calcium's entry is the initial cytosolic event, preceding troponin's action, site exposure, and cross-bridge formation. This order, rooted in excitation-contraction coupling, ensures contraction starts with a neural signal, distinguishing it from subsequent mechanical steps.

Correct Answer: D

Rationale: Smooth muscle, found in organ walls, lacks striations due to unaligned actin and myosin, operates involuntarily under autonomic control, and has one nucleus per cell. Skeletal muscle is striated, voluntary, and multinucleate; cardiac muscle is striated, involuntary, and uninucleate with intercalated discs. Smooth muscle's non-striated, single-nucleus, involuntary nature suits its role in visceral functions like digestion, distinguishing it from skeletal and cardiac types in structure and regulation.