A woman complains of problems going up steps. Tests reveal hip extension weakness but no issues with hip flexion or knee flexion or extension. What muscle is most likely damaged?

Correct Answer: B

Rationale: Hip extension, critical for climbing steps, is primarily powered by the gluteus maximus, which extends the thigh backward. Weakness here impairs stair ascent, while intact hip flexion (iliopsoas) and knee flexion/extension (hamstrings/quadriceps) suggest a specific issue. Adductor magnus adducts, not extends. Gluteus medius abducts and stabilizes. Semitendinosus flexes the knee, not hip extension. Gluteus maximus is the correct answer, as its damage aligns with isolated hip extension weakness, a key mover in upward locomotion.

Correct Answer: C

Rationale: Muscle types are cardiac, smooth, and skeletal, defined by structure and function. Cardiac drives the heart, smooth lines organs, and skeletal moves bones. Sesamoids are small bones embedded in tendons, like the patella, not muscles they enhance leverage, not contract. Including them as muscle misrepresents anatomy, as they're skeletal components. The three muscle types cover all contractile tissues, while sesamoids support mechanically, lacking muscle's cellular traits, making them the clear non-muscle outlier in this context.

Correct Answer: D

Rationale: Myoglobin, a red pigment in muscle fibers, binds and stores oxygen, releasing it during activity when blood supply can't meet demand, like in intense exercise. This supports aerobic respiration in mitochondria, especially in slow-twitch fibers. Hemoglobin, also red, transports oxygen in blood, not storing it in muscles. Erythrocytes are red blood cells carrying hemoglobin, not pigments themselves. Sarcoplasm is the muscle cell's cytoplasm, not a pigment or oxygen store. Myoglobin's muscle-specific oxygen storage distinguishes it, enhancing endurance by buffering oxygen availability, unlike hemoglobin's circulatory role or the non-storage nature of sarcoplasm and erythrocytes, aligning with its biochemical function in muscle tissue.

Correct Answer: A

Rationale: Muscle fatigue sets in when ATP depletes, halting cross-bridge cycling as myosin can't detach or re-cock without energy, despite calcium presence. This follows prolonged activity outpacing ATP regeneration. The latent period, pre-contraction, involves signal delay, not fatigue. Relaxation begins as calcium returns to the SR, not ATP exhaustion. Lactic acid breakdown isn't a fatigue marker it accumulates, not depletes, during anaerobic effort. ATP shortage directly impairs contraction, distinguishing fatigue from timing phases or metabolic byproducts, reflecting energy failure's impact on muscle performance.

Correct Answer: B

Rationale: The flexor carpi ulnaris flexes the wrist on the ulnar (medial) side, bending it toward the forearm's little-finger edge. 'Flexor' indicates bending, 'carpi' targets the wrist, and 'ulnaris' specifies the ulna's side. Flexing or extending the ulna itself misinterprets bones don't flex, joints do. Extending the wrist contradicts 'flexor.' Its action aligns with wrist movement, distinct from bone or opposite motions, key for hand positioning.