The release of ADP and P from the myosin head causes the

Correct Answer: D

Rationale: When ADP and inorganic phosphate (P) release from myosin's head after ATP hydrolysis, the cross-bridge bends, pulling actin toward the sarcomere's center the power stroke. This single event encompasses bridge bending, filament pulling, and the stroke itself, driving contraction. Each aspect isn't separate; they're facets of one action. Bending alone omits the pull, pulling alone skips mechanics, and power-stroke alone undersells the full process. All occur simultaneously as myosin resets, distinguishing this step from prior ATP binding or later detachment, critical for the sliding filament cycle and muscle shortening.

Correct Answer: B

Rationale: External intercostals lift ribs during inspiration, expanding the chest for inhalation. Internal intercostals depress ribs in forced expiration, aiding exhalation. Opposite pairing misaligns their roles. Trunk flexion or extension involves abdominals or back muscles, not intercostals. Their respiratory functions distinguish them, critical for breathing mechanics, unlike trunk movement roles.

Correct Answer: B

Rationale: Medial thigh muscles, like adductor longus, pull the thigh toward the midline, adducting it, as in crossing legs. Abduction moves it outward, a lateral thigh role. Leg extension (knee) or flexion involves quadriceps or hamstrings, not medial focus. Adduction defines their action, distinct from lateral, knee, or bending functions, key for thigh stability.

Correct Answer: C

Rationale: At the neuromuscular junction, acetylcholine release initiates muscle contraction by binding to receptors on the muscle membrane, causing depolarization and an action potential that propagates along the fiber. This triggers calcium release from the sarcoplasmic reticulum, which binds to troponin, shifting tropomyosin to expose myosin-binding sites on actin, enabling contraction. These events form a direct cascade from acetylcholine's action. However, ATP binding to the myosin head occurs later in the cross-bridge cycle, detaching myosin from actin after a power stroke, a process not directly initiated by acetylcholine but part of the contraction-relaxation cycle powered by ATP hydrolysis. This distinction highlights that while calcium and actin exposure are immediate downstream effects, ATP's role is a subsequent step, reliant on energy dynamics rather than the initial synaptic signal, making it the exception in this sequence.

Correct Answer: D

Rationale: Shoulder muscles, like deltoid, supraspinatus, and teres minor, act on the shoulder joint. Deltoid abducts the arm, supraspinatus initiates abduction, and teres minor rotates it, all part of the rotator cuff or girdle musculature. Pectineus, in the thigh, flexes and adducts the hip, not the shoulder it's a medial thigh muscle. Subscapularis, another rotator cuff muscle, was replaced here to fit four options, but pectineus remains the outlier. Its hip-focused action contrasts with shoulder-specific roles, distinguishing it as unrelated to shoulder movement or stability in the musculoskeletal system.