The large muscle group that attaches the leg to the pelvic girdle and produces extension of the hip joint is the group.

Correct Answer: A

Rationale: Gluteal muscles (gluteus maximus, medius, minimus) attach the leg to the pelvic girdle, with maximus extending the hip (e.g., standing up). Obturators rotate the thigh. Adductors pull it inward. Abductors (e.g., gluteus medius) move it outward, not extending. Gluteals' size and extension role distinguish them, key for hip motion.

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.

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.