Which is the largest of the structures in a muscle fibre?

Correct Answer: A

Rationale: Muscle fibres contain hierarchical structures. Myofibrils, bundles of sarcomeres, span the fibre's length (tens to hundreds of micrometres), housing myofilaments actin and myosin arrays within sarcomeres. Myosin is a protein molecule, and myopic is unrelated (likely a typo). Myofibrils, as the largest organized unit within the cell, integrate sarcomeres for contraction, distinguishing them from shorter myofilaments or molecular components, critical for muscle's macroscopic force generation.

Correct Answer: D

Rationale: Skeletal muscle's multinucleate nature a syncytium from fused myoblasts contrasts with uninucleate cardiac and smooth muscles. Striations occur in skeletal and cardiac, branching and intercalated discs are cardiac-specific. Multiple nuclei support skeletal muscle's size and repair, distinguishing it in structure and voluntary function, key to its histological identity.

Correct Answer: B

Rationale: Gluteus medius indicates size intermediate between maximus and minimus among gluteal muscles. Sternocleidomastoid names origin-insertion, flexor digitorum profundus action-depth, and trapezius shape. Medius' size-based naming distinguishes it, reflecting relative scale in gluteal anatomy, useful for comparative identification.

Correct Answer: B

Rationale: Thin myofilaments comprise actin (structure), tropomyosin (covers binding sites), and troponin (calcium receptor), regulating contraction. Thick myofilaments are myosin only, with heads forming cross-bridges not binding sites and lack tropomyosin/troponin. Myosin isn't in thin filaments. This composition distinguishes thin filaments' regulatory role, critical for controlled muscle action, contrasting with thick filaments' motor function.

Correct Answer: C

Rationale: Sarcomeres, within myofibrils, contract by sliding actin and myosin, shortening the myofibril and thus the cell. Sarcoplasm houses organelles, sarcolemma conducts signals, and sarcoplasmic reticulum stores calcium not contractile. Sarcomeres' role as the contractile engine distinguishes them, essential for muscle movement mechanics.