Identify the muscles that form the rotator cuff.

Correct Answer: A

Rationale: The rotator cuff comprises infraspinatus, supraspinatus, teres minor, and subscapularis, stabilizing the shoulder joint and enabling rotation. 'Intraspinatus' is a typo correct is infraspinatus. Teres major and deltoid assist shoulder movement but aren't cuff muscles. These four anchor the humerus, distinct from larger or adjacent muscles, vital for shoulder integrity and motion.

Correct Answer: D

Rationale: Muscle contraction and relaxation drive movement through a coordinated process. Contraction occurs when nerve impulses trigger calcium release, enabling myosin heads to bind actin and pull filaments together, shortening the muscle and moving attached structures, like bones in skeletal muscle. Relaxation follows when calcium is pumped back into the sarcoplasmic reticulum, halting myosin-actin interaction, allowing the muscle to lengthen and rest. The incomplete option likely misrepresents a molecular detail (e.g., 'Xn' for ions or proteins), but lacks clarity. Cell layer thickness affects force, not the process itself. Vessel linings support circulation, not contraction mechanics. The contraction-relaxation cycle, powered by ATP and regulated by calcium, is the correct mechanism, universally applicable to skeletal, cardiac, and smooth muscle functions, making it the accurate statement despite the truncated option.

Correct Answer: B

Rationale: Type IIb (or IIx in humans) muscle fibers are fast-twitch, with high motor unit strength and fast contractile speed, suited for explosive efforts like sprinting, but they have low oxidative capacity, relying on anaerobic glycolysis, leading to rapid fatigue. High fatigue resistance, typical of slow-twitch (Type I) fibers with rich mitochondria and myoglobin, doesn't fit Type IIb's profile they tire quickly due to limited aerobic endurance. Low oxidative capacity aligns with their anaerobic nature, and strength and speed reflect their power. Fatigue resistance misaligns with their design, distinguishing them from endurance-focused fibers, key to understanding muscle fiber specialization.

Correct Answer: B

Rationale: Flexion decreases the angle between bones, as in bending the elbow (humerus and radius/ulna), bringing bones closer via flexor muscles like biceps. Increasing the angle occurs in extension, the opposite motion. Moving away from the body describes abduction, not flexion, which is joint-specific. Moving toward the body's center could fit some flexions (e.g., hip), but it's less universal than angle decrease, which defines flexion across joints like elbow, knee, or neck, distinguishing it from extension, lateral, or midline motions.

Correct Answer: B

Rationale: The inferior oblique, an extraocular muscle, resides in the eye socket, originating near the orbit's front and inserting on the eyeball, elevating and abducting it. Abdominal muscles (e.g., obliques) flex the trunk. Anterior neck hosts muscles like sternocleidomastoid. Facial muscles (e.g., zygomaticus) move skin. The inferior oblique's orbital location and eye movement role distinguish it, essential for gaze, unlike trunk, neck, or facial functions.