The primary muscle of breathing is the:

Correct Answer: A

Rationale: The diaphragm, a dome-shaped muscle below the lungs, contracts to flatten, expanding the thoracic cavity for inspiration breathing's primary driver. External intercostals assist, lifting ribs, but diaphragm's action is dominant. Internal intercostals aid expiration, secondary to inspiration. Rectus abdominis flexes the trunk, not breathing. Diaphragm's central role in volume change sets it apart, essential for respiratory rhythm.

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: C

Rationale: The anterior cranial fossa, housing the frontal brain lobes, is formed by the orbital part of the frontal bone (roof), ethmoid bone (midline, cribriform plate), and sphenoid bone (lesser wings, posteriorly). These create a shallow depression. The temporal bone, lateral and forming the middle fossa, doesn't contribute anteriorly its squamous and petrous parts support the temporal lobes instead. The frontal bone's orbital plate, ethmoid's midline, and sphenoid's wings align with the fossa's anatomy, while the temporal's exclusion reflects its distinct cranial position, critical for understanding skull compartmentalization.

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.