Which of the following is a prime mover in head flexion?

Correct Answer: C

Rationale: Sternocleidomastoid, from sternum/clavicle to mastoid, flexes the head forward when both sides contract, a prime mover in neck flexion. Occipitofrontalis raises eyebrows and wrinkles forehead, not flexing. Corrugator supercilii furrows brows, a facial expression muscle. Masseter elevates the mandible for chewing, not head flexion. Sternocleidomastoid's bilateral action drives head bending, distinguishing it from facial or jaw muscles, critical for neck movement.

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 diaphragm's upward movement, as in exhalation, reduces chest volume. Per Boyle's law, this increases pressure in the lungs, forcing air out as the higher internal pressure exceeds atmospheric pressure. Inhalation occurs when the diaphragm descends, expanding volume and lowering pressure, drawing air in. The upward push compresses the chest, driving exhalation, a fundamental respiratory mechanism ensuring gas exchange, distinct from inhalation's volume increase and pressure drop.

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.