Identify a muscle that promotes smiling.

Correct Answer: D

Rationale: Zygomaticus (major and minor) lifts the mouth's corners upward and laterally, forming a smile. Buccinator compresses cheeks, not smiling. Frontalis raises the brow, unrelated to lips. Orbicularis oris purses lips, as in kissing, not smiling. Zygomaticus' specific pull on the mouth defines smiling, distinct from cheek, brow, or lip-closing actions, central to happy expressions.

Correct Answer: A

Rationale: Gastrocnemius, on the posterior leg, forms the calf's bulk, plantarflexing the foot, as in walking. Semitendinosus, a hamstring, is thigh-based. Tibialis anterior, anterior leg, dorsiflexes. Vastus lateralis, a quadriceps, is thigh-front. Gastrocnemius' calf prominence and action set it apart, key for locomotion.

Correct Answer: D

Rationale: Muscle cells, or fibers, enable movement through their ability to contract and relax, a process driven by actin and myosin interactions. In skeletal muscles, nervous system signals trigger contraction: myosin heads pull actin filaments, shortening the muscle and moving attached bones. Relaxation follows when signals cease, allowing fibers to lengthen, returning bones to rest. This applies to voluntary actions (skeletal muscle), heart pumping (cardiac), and organ functions (smooth muscle). Blood flow delivers oxygen and nutrients, supporting energy production, but doesn't directly cause movement. Cell layer thickness influences strength, not the mechanism. Vessel linings (endothelium) regulate flow, not muscle action. Contraction and relaxation are the core processes, fueled by ATP and coordinated by nerves, making them the fundamental drivers of bodily movement across muscle types.

Correct Answer: B

Rationale: The coracoid process, a hook-like projection on the scapula, extends anteriorly below the clavicle, anchoring muscles like pectoralis minor and biceps. It stabilizes the shoulder joint. The humerus, articulating with the scapula, lacks this process. The femur, in the thigh, supports leg movement, not shoulder. The pelvic girdle (ilium, ischium, pubis) forms the hip, unrelated to upper limb anatomy. The scapula's coracoid is distinct, critical for shoulder mechanics, unlike arm, leg, or pelvic structures, reflecting its upper body role.

Correct Answer: D

Rationale: Facial expression muscles, like orbicularis oris or zygomaticus, uniquely insert into the skin, allowing fine movements like smiling or frowning, unlike skeletal muscles attaching to bones via tendons. They don't all originate from the scalp many arise from facial bones (e.g., zygomaticus from zygomatic bone). Cartilage insertions are rare and incorrect facial cartilage (e.g., nasal) isn't a primary target. Insertion solely onto facial bones applies to some but not all (e.g., masseter), missing the skin's role. Skin insertion enables their role in subtle, dynamic expressions, distinguishing them from scalp-origin, cartilage, or bone-only attachments, key to facial muscle function.