A red pigment that stores oxygen for muscle use is

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: 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.

Correct Answer: B

Rationale: Deltoid abducts the arm horizontally, lifting it sideways from the body, due to its lateral fibers' orientation. Biceps brachii flexes the elbow. Trapezius elevates or adducts scapula. Triceps brachii extends the elbow. Deltoid's abduction role, centered at the shoulder, sets it apart, vital for arm positioning, distinct from elbow or scapular functions.

Correct Answer: C

Rationale: Quadriceps extend the knee, straightening the leg, while hamstrings flex it, bending it, acting as antagonists at the knee joint. Abduction-adduction involves lateral-medial thigh motion, not their primary knee focus. Extension-flexion defines their opposing actions, distinct from side movements, crucial for leg dynamics.

Correct Answer: B

Rationale: Sarcomere shortening, the basis of muscle contraction, occurs via the sliding filament mechanism. Myosin heads, protruding from thick filaments, bind to actin on thin filaments, forming cross-bridges. Using ATP-derived energy, these heads execute a power stroke, pulling actin toward the sarcomere's center specifically the A band's midpoint reducing the distance between Z lines and shortening the sarcomere. Acetylcholine initiates this by triggering an action potential, but it's not the direct shortening mechanism. Calcium release from the sarcoplasmic reticulum enables myosin-actin binding by shifting tropomyosin, a crucial step but not the act of shortening. ATP breakdown powers the process, yet the sarcoplasmic reticulum stores calcium, not hydrolyzes ATP for shortening. The myosin-actin interaction is the mechanical driver, making it the precise answer, central to muscle movement and the sliding filament theory.