Which myofilament has cross-bridges?

Correct Answer: C

Rationale: Myosin, the thick filament in sarcomeres, features cross-bridges protruding heads that bind actin during contraction. These bridges, powered by ATP, pull actin inward, driving the power stroke. Troponin, on thin filaments, binds calcium to regulate contraction, lacking bridges. Actin forms thin filaments, receiving cross-bridges, not bearing them. Tropomyosin shields actin's sites, also without bridges. Myosin's cross-bridges are unique, enabling force generation, distinguishing it from actin's structural role or troponin and tropomyosin's regulatory functions, essential for the sliding filament mechanism and muscle movement.

Correct Answer: D

Rationale: The sternocleidomastoid, from sternum and clavicle to mastoid, rotates and tilts the head side to side, as in looking over the shoulder. Arm abduction is deltoid's role. Breathing involves diaphragm and intercostals, not this neck muscle. Shoulder shrugging is trapezius. Its head-turning action distinguishes it, vital for neck mobility, unlike arm, respiratory, or shoulder functions.

Correct Answer: D

Rationale: Biceps brachii flexes the elbow, bending the forearm toward the arm, while triceps brachii extends it, straightening it. Abduction and adduction involve lateral movement, not their primary elbow focus. Flexion-extension defines their antagonistic pair, distinct from side motions, key for arm bending and straightening.

Correct Answer: D

Rationale: Muscular dystrophy encompasses genetic disorders like Duchenne, progressively weakening muscles via protein defects, leading to loss of function. Sprains are ligament injuries. Fibromyalgia is pain without degeneration. Tendonitis inflames tendons. Muscular dystrophy's degenerative nature distinguishes it, impacting muscle integrity over time.

Correct Answer: B

Rationale: The acromion process is a bony projection on the scapula, forming the shoulder's highest point where it articulates with the clavicle at the acromioclavicular joint. This structure stabilizes the shoulder girdle and anchors muscles like the deltoid. The ilium, part of the pelvis, supports hip muscles, not shoulder features. The humerus, the upper arm bone, articulates with the scapula at the glenoid cavity, not bearing the acromion. The ulna, a forearm bone, connects to the humerus distally, unrelated to shoulder anatomy. The scapula's acromion is distinct, critical for shoulder movement and stability, unlike pelvic, arm, or forearm bones, aligning with its role in the musculoskeletal framework.