Smiling broadly is difficult after dental anesthesia. Which muscle is most likely affected?

Correct Answer: B

Rationale: Smiling broadly involves the zygomaticus major, which pulls the mouth corners upward and laterally. Dental anesthesia, often targeting the facial nerve branches, can paralyze this muscle, making smiling difficult. Orbicularis oris purses the lips, not elevating them for smiling. Levator labii superioris lifts the upper lip, less critical for broad smiles. Risorius widens the mouth but is secondary to zygomaticus major. Zygomaticus major is the correct answer, as it's the primary smiling muscle affected by facial nerve anesthesia, evident in post-dental asymmetry or weakness.

Correct Answer: D

Rationale: Cardiac muscle resides solely in the heart, forming its walls to enable relentless, involuntary contractions that pump blood throughout life. Its striated, branched structure and autonomic control distinguish it from other muscles. Skin contains no muscle tissue, only epithelial and connective layers. Lungs rely on smooth muscle for airway regulation, not cardiac. Bones are skeletal muscle anchors, not muscle sites. The heart's unique cardiac muscle sustains circulation, tireless and self-regulating, unlike smooth muscle's visceral roles or skeletal muscle's voluntary actions. This specificity underscores its critical function, driving the cardiovascular system, making its location unmistakable and vital, separate from unrelated tissues or organs misaligned with its purpose.

Correct Answer: A

Rationale: A stretch reflex occurs when a muscle is passively stretched, triggering a rapid contraction to resist the stretch, mediated by muscle spindles and a monosynaptic reflex arc. The classic example is the knee-jerk response, where tapping the patellar tendon stretches the quadriceps, causing an immediate kick. This reflex tests spinal nerve function and is widely recognized in clinical settings. A tendon reflex, while related to tension, typically involves Golgi tendon organs inhibiting contraction, not initiating it. A flexor reflex is a polysynaptic withdrawal response to pain, not passive stretch. An ipsilateral reflex occurs on the same side but isn't specific to stretch. The knee-jerk response perfectly exemplifies a stretch reflex due to its simplicity and direct muscle activation, making it the best fit, as it's a foundational concept in neurophysiology for assessing reflex integrity.

Correct Answer: B

Rationale: In a sarcomere, the contractile unit of muscle, thick filaments are composed of myosin, a protein with cross-bridges that pull actin during contraction. This forms the dark A band, driving the sliding filament mechanism. Actin forms thin filaments, interacting with myosin, not constituting thick ones. Myoglobin stores oxygen, unrelated to filament structure. Tropomyosin regulates actin's active sites on thin filaments, not part of thick filaments. Myosin's structural and functional role in thick filaments is key, enabling force generation, distinct from actin's thin filament role, myoglobin's metabolic support, or tropomyosin's regulatory function, fundamental to muscle contraction mechanics.

Correct Answer: B

Rationale: Post-exercise deep breathing repays oxygen debt the oxygen deficit incurred when demand exceeds supply during intense activity. This restores oxygen to myoglobin, oxidizes lactic acid back to pyruvate, and regenerates ATP via aerobic respiration. Fatigue is a symptom, not the cause of breathing. Lactic acid accumulation contributes to debt but isn't the full reason oxygen replenishes broader systems. Combining them excludes fatigue's role, missing the debt's primacy. Oxygen debt drives this response, addressing metabolic recovery, distinct from fatigue's effect or lactic acid's partial role, key to post-exercise homeostasis.