Which one of the following statements regarding muscle contraction and relaxation is correct?

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: 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.

Correct Answer: D

Rationale: Sartorius, a long, strap-like muscle, flexes, abducts, and laterally rotates the hip, then flexes the knee, enabling leg crossing. Gluteus maximus extends the hip. Piriformis rotates it. Gracilis adducts, not crossing fully. Sartorius' multi-joint action distinguishes it, critical for this coordinated motion.

Correct Answer: C

Rationale: Intestinal symptoms like diarrhea, vomiting, and abdominal pain stem from toxins disrupting gut function. Enterotoxins, produced by bacteria such as Escherichia coli or Staphylococcus aureus, target intestinal cells, increasing secretion or reducing absorption by altering ion channels or tight junctions, as seen in cholera. Cytotoxins cause broader cell damage, not specific to the gut. Neurotoxins affect nerves, not digestion directly. Leukocidins attack immune cells, not intestinal ones. Enterotoxins' specific action on gut epithelium, leading to fluid loss and characteristic symptoms, distinguishes them as the cause, critical for diagnosing gastrointestinal bacterial infections.