In the process of muscle contraction, what happens when ATP binds to the myosin head?

Correct Answer: C

Rationale: During muscle contraction, the cross-bridge cycle drives filament sliding. After myosin binds to actin and performs a power stroke pulling actin toward the sarcomere's center ATP binds to the myosin head. This binding reduces myosin's affinity for actin, breaking the cross-bridge and detaching myosin, allowing it to reset for another cycle. ATP is then hydrolyzed to ADP and phosphate, cocking the head into a high-energy state to reattach to actin. Calcium ions, managed by the sarcoplasmic reticulum, regulate actin exposure, not myosin detachment. Acetylcholine release occurs at the neuromuscular junction, unrelated to ATP's role here. Hydrolysis energizes myosin but doesn't cause attachment binding does the detachment. This ATP-driven detachment is critical for repeated contractions, distinguishing it from initiation or calcium-related steps, and underscores its mechanical role in muscle dynamics.

Correct Answer: D

Rationale: Endurance training enhances intramuscular lipid use during exercise, increasing storage and oxidation efficiency as mitochondria and enzymes adapt, sparing glycogen for longer efforts. Lowering intramuscular lipid storage contradicts this training boosts it for fuel. Increasing liver glycogen content occurs, but it's not the primary exercise-use adaptation; it supports glucose homeostasis. Increasing muscle glycogen use during exercise opposes training's glycogen-sparing effect use efficiency improves, not quantity. Intramuscular lipid reliance grows, distinguishing it as a key adaptation, aligning with endurance's fat-burning shift, unlike glycogen-focused or storage-reducing changes.

Correct Answer: A

Rationale: A bipennate muscle, like rectus femoris, has fibers angling into a central tendon from both sides, resembling a feather's two-sided barbs, boosting force output. All-direction angling fits convergent patterns (e.g., pectoralis major), not bipennate. One-sided angling describes unipennate (e.g., tibialis posterior). Opposite angles misrepresent bipennate fibers align similarly on both sides. This dual-sided structure maximizes fiber packing and strength, distinguishing it from multidirectional, single-sided, or misangled patterns, critical for powerful joint actions.

Correct Answer: B

Rationale: The linea alba, a midline fibrous band, runs vertically, splitting rectus abdominis into left and right halves, formed by aponeuroses of abdominal muscles. It's tendinous, not muscular, and doesn't compress organs obliques do that. It's not a hip-knee connector (e.g., iliotibial band) or tendinous inscription (rectus abdominis crossbands). Its central tendon role distinguishes it, vital for abdominal wall unity.

Correct Answer: C

Rationale: The kidney's layered anatomy includes the cortex as the outer region, the renal pelvis as the innermost collecting area, and the renal medulla as the intermediate zone. The medulla, situated between the cortex and pelvis, contains renal pyramids triangular structures that channel urine toward the pelvis via collecting ducts. Unlike the cortex, which filters blood, the medulla concentrates urine, regulating water and electrolyte balance through its loop of Henle and collecting tubules. The nephron is a functional unit crossing multiple layers, not a layer itself, while the renal pelvis is a central cavity. The medulla's middle position and its role in urine concentration distinguish it, reflecting the kidney's progressive processing from filtration to excretion.