Which of the following is the smallest structure within a muscle fibre?

Correct Answer: A

Rationale: Muscle fibres contain nested structures. Myosin, a protein molecule, forms thick myofilaments bundles of myosin within sarcomeres, the contractile units. Sarcomeres align in myofibrils, spanning the fibre. Myosin, at the molecular level (about 160 nm long), is smaller than myofilaments (micrometres), sarcomeres (2 micrometres), and myofibrils (cell-length). Its role as a building block for thicker structures marks it as the smallest, foundational to contraction mechanics, distinguishing it from larger assemblies in muscle hierarchy.

Correct Answer: B

Rationale: A fascicle, a bundle of muscle fibres (cells), is larger than a single fibre, which contains myofibrils bundles of sarcomeres. Sarcomeres, the smallest contractile units, are within myofibrils. Fascicles, wrapped in perimysium, contribute to whole-muscle structure, distinguishing them as the largest listed unit, key to muscle organization and force production.

Correct Answer: B

Rationale: Triceps brachii indicates location ( brachii, arm) and origins ( tri-, three). Sartorius is whimsical (tailor's muscle), soleus reflects shape (sandal), and trapezius shape (trapezoid). Brachii's arm-specificity marks it as location-named, distinguishing it from action or whimsical terms, aiding anatomical clarity.

Correct Answer: A

Rationale: Increasing the angle between finger and hand bones means finger extension straightening from a flexed position. Extensor digitorum, on the forearm's dorsal side, contracts to pull finger phalanges, extending them at metacarpophalangeal joints. Extensor carpi ulnaris extends the wrist, flexor digitorum profundus flexes fingers, and abductor pollicis longus moves the thumb. Extensor digitorum's specific finger-extension role distinguishes it, essential for opening the hand, contrasting with wrist or thumb actions.

Correct Answer: B

Rationale: Size hierarchy: muscle (organ) contains fasciculi (fibre bundles), then muscle fibres (cells), then myofibrils (fibre-length). Sarcomeres (2 micrometres) and myofilaments are within myofibrils, myosin is molecular. Only 'muscle, fasciculus, fibre, myofibril' descends correctly, distinguishing organizational scale, key to muscle structure understanding.