Where cardiac muscle is found?

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: 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.

Correct Answer: B

Rationale: The frontalis, on the forehead, wrinkles the brow and raises eyebrows, creating horizontal lines, as in surprise. Buccinator compresses cheeks, aiding chewing. Orbicularis oculi closes eyes, wrinkling skin around them, not the brow. Zygomaticus lifts mouth corners for smiling. Frontalis' forehead action distinguishes it, key for facial expression above the eyes.

Correct Answer: C

Rationale: Pectoralis major, across the chest, adducts and medially rotates the arm, pulling it toward the midline, as in hugging. Biceps brachii flexes the elbow. Latissimus dorsi adducts from behind, not chest-directed. Triceps brachii extends the elbow. Pectoralis major's anterior pull distinguishes it, key for chest-centric motion, unlike elbow or posterior actions.