Identify muscles of mastication.

Correct Answer: B

Rationale: Masseter and temporalis are primary mastication muscles, closing the jaw for chewing. Masseter elevates the mandible, temporalis assists and retracts. Buccinator aids chewing indirectly via cheeks, not jaw movement. Orbicularis oris moves lips, unrelated. Zygomaticus smiles, not chews. These two excel in jaw power, distinct from accessory, lip, or facial roles, essential for biting and grinding.

Correct Answer: C

Rationale: Fibromyalgia features chronic muscle pain, tenderness, stiffness, and elevated substance P, a pain neurotransmitter, with no clear cause. Cancer may cause pain but isn't muscle-specific or tied to substance P. Diabetes affects nerves, not primarily muscles. Tetanus causes spasms from infection, not chronic ache. Fibromyalgia's diffuse, persistent symptoms set it apart, a musculoskeletal disorder.

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.