ATP is first regenerated by

Correct Answer: C

Rationale: After ATP depletion in muscle contraction, creatine phosphate rapidly donates a phosphate to ADP, regenerating ATP in seconds via creatine kinase. This immediate source sustains short bursts of activity. Cellular respiration, in mitochondria, follows later, using oxygen for sustained ATP but takes time. Lactic acid breakdown isn't a process lactic acid forms in anaerobic conditions, not regenerating ATP. Fermentation (anaerobic glycolysis) produces ATP slowly, secondary to creatine phosphate. Its quick, oxygen-independent action distinguishes creatine phosphate, bridging energy gaps, unlike slower respiration or fermentation, vital for initial muscle energy demands.

Correct Answer: A

Rationale: Rectus abdominis, along the abdomen, flexes the vertebral column, bending the spine forward, as in sit-ups. Serratus anterior protracts scapula. Sternocleidomastoid flexes the neck, not full column. Trapezius extends the head or adducts scapula. Rectus abdominis' anterior pull distinguishes it, key for trunk flexion, unlike scapular or neck-specific actions.

Correct Answer: B

Rationale: Quadriceps occupy the anterior thigh, comprising four muscles (rectus femoris, vastus lateralis, medialis, intermedius) that extend the knee. Anterior abdomen hosts rectus abdominis. Posterior thigh has hamstrings. Posterior arm holds triceps. Quadriceps' thigh-front location sets them apart, key for leg movement, distinct from other regions.

Correct Answer: B

Rationale: The A band in a sarcomere represents the region containing thick myosin filaments, overlapping with thin actin filaments in striated muscle. During contraction, the sliding filament theory explains that actin slides over myosin, shortening the sarcomere as Z lines draw closer. However, the A band's length defined by the fixed length of myosin filaments remains constant. The I band (actin-only) and H zone (myosin-only) shrink as overlap increases, but the myosin span doesn't change, keeping the A band stable. Z lines anchor actin, not define the A band, and actin-only regions are the I band, not A. This unchanged A band length is a key feature in muscle mechanics, reflecting the structural integrity of myosin and the dynamic overlap driving contraction, a fundamental concept in understanding sarcomere function.

Correct Answer: B

Rationale: Cervical vertebrae (C1-C7) differ from thoracic (T1-T12). They have smaller bodies, supporting the lighter head versus the thorax's trunk. Their spinous processes are shorter (except C7), aiding neck mobility, unlike thoracic's longer, downward-slanting ones for muscle attachment. Transverse processes are shorter but feature foramina transversaria for arteries, unlike thoracic's longer ones. However, cervical vertebrae have larger vertebral foramina to accommodate the spinal cord's cervical enlargement, not smaller this supports greater nerve output for upper limbs. The false statement is smaller foramina, as cervical design prioritizes cord space, distinguishing it from thoracic structure.