The dark region of a sarcomere is called the

Correct Answer: C

Rationale: The A band, the dark region of a sarcomere, spans the thick myosin filaments, appearing dark under a microscope due to overlapping actin and myosin in most areas. It remains constant in length during contraction, reflecting myosin's extent. The H-zone, within the A band, is lighter, showing only myosin, narrowing as actin slides in. The Z-line anchors actin, defining sarcomere boundaries, not dark. The I band, light, contains only actin, shortening during contraction. The A band's darkness and myosin content distinguish it, central to sarcomere structure, unlike the lighter, shifting H-zone, structural Z-line, or actin-only I band, key to understanding muscle striation.

Correct Answer: C

Rationale: The depressor anguli oris pulls the mouth's corners down, and depressor labii inferioris lowers the bottom lip, both creating frowning or pouting expressions. Chewing involves masseter and temporalis. Cheek compression is buccinator's role. Smiling uses zygomaticus. These depressors' downward actions contrast with chewing, compression, or upward smiling, key for sad or sulky faces.

Correct Answer: B

Rationale: Spasms are involuntary, sudden muscle contractions, often brief and uncontrolled, like twitches. Cramps are painful, prolonged involuntary contractions, a subset but not the broad term. Strains are muscle overstretching injuries, not contractions. Sprains affect ligaments, not muscles. Spasms' involuntary nature encompasses various unintended movements, distinguishing them from specific pain, injury, or ligament issues, common in muscle dysfunction.

Correct Answer: C

Rationale: A strain is a muscle stretch or tear from overexertion, like lifting too heavy. Cramps are painful, involuntary contractions. Spasms are sudden contractions, not tears. Sprains affect ligaments. Strain's injury focus distinguishes it, common in sports, unlike contraction-based or ligament issues.

Correct Answer: B

Rationale: The clavicle is a slender, S-shaped bone extending horizontally from the sternum to the scapula, forming part of the shoulder girdle. Known as the collar bone, it links the arm to the axial skeleton, providing support and mobility while protecting underlying vessels and nerves. Its superficial position above the chest makes it palpable and prone to fracture. The shoulder blade is the scapula, a flat, triangular bone posteriorly. The jaw bone is the mandible, unrelated to the shoulder. The breast bone, or sternum, is the central chest bone connecting ribs. The clavicle's distinct role in shoulder articulation and its colloquial 'collar bone' name reflecting its necklace-like placement clearly identify it, distinguishing it from other skeletal structures in anatomical context.