What is the basic functional unit of skeletal muscle tissue?

Correct Answer: B

Rationale: The sarcomere is the basic functional unit of skeletal muscle tissue, defined as the segment between two Z-lines where actin and myosin filaments overlap to produce contraction via the sliding filament mechanism. This repeating unit shortens during muscle action, driving movement. Muscle fibers, single cells containing many sarcomeres, are structural units, not the smallest functional ones. Myofibrils, bundles of sarcomeres within fibers, organize contraction but aren't the base unit sarcomeres are their components. The sarcoplasmic reticulum stores calcium, enabling contraction, but doesn't contract itself. The sarcomere's role as the contractile core distinguishes it, fundamental to muscle function, unlike larger structural or supportive elements, aligning with its physiological definition.

Correct Answer: C

Rationale: Sternocleidomastoid, from sternum/clavicle to mastoid, flexes the head forward when both sides contract, a prime mover in neck flexion. Occipitofrontalis raises eyebrows and wrinkles forehead, not flexing. Corrugator supercilii furrows brows, a facial expression muscle. Masseter elevates the mandible for chewing, not head flexion. Sternocleidomastoid's bilateral action drives head bending, distinguishing it from facial or jaw muscles, critical for neck movement.

Correct Answer: A

Rationale: Gluteal muscles (gluteus maximus, medius, minimus) attach the leg to the pelvic girdle, with maximus extending the hip (e.g., standing up). Obturators rotate the thigh. Adductors pull it inward. Abductors (e.g., gluteus medius) move it outward, not extending. Gluteals' size and extension role distinguish them, key for hip motion.

Correct Answer: A

Rationale: Toxins vary in their cellular targets and effects. Cytotoxins directly harm cells by disrupting membranes, inhibiting protein synthesis, or inducing apoptosis, leading to tissue damage or organ dysfunction, as seen with diphtheria toxin. Neurotoxins target nerve cells, impairing signaling, like botulinum toxin. Enterotoxins affect intestinal cells, causing symptoms like diarrhea, as with cholera toxin. Leukocidins destroy white blood cells, aiding bacterial evasion. Cytotoxins' broad cellular toxicity, applicable across cell types, contrasts with the specialized actions of others, making them the primary agents of direct cell damage in this context.

Correct Answer: B

Rationale: The diaphragm's upward movement, as in exhalation, reduces chest volume. Per Boyle's law, this increases pressure in the lungs, forcing air out as the higher internal pressure exceeds atmospheric pressure. Inhalation occurs when the diaphragm descends, expanding volume and lowering pressure, drawing air in. The upward push compresses the chest, driving exhalation, a fundamental respiratory mechanism ensuring gas exchange, distinct from inhalation's volume increase and pressure drop.