What is the role of calcium ions in the sliding filament theory of contraction?

Correct Answer: B

Rationale: In the sliding filament theory, calcium ions bind to troponin on thin filaments, causing a conformational shift that moves tropomyosin, exposing actin's myosin-binding sites. This enables cross-bridges to form, initiating contraction. Calcium doesn't bind myosin directly myosin's activation relies on ATP and site exposure. It also doesn't hydrolyze ATP ATP binds myosin for that. 'All of the above' overextends calcium's role. Its specific binding to troponin is pivotal, triggering the cascade for actin-myosin interaction, distinct from myosin's mechanics or ATP's energy role, central to contraction's molecular choreography.

Correct Answer: D

Rationale: Trapezius adducts the scapula, pulling it toward the spine, and extends the head backward when both sides contract. Buccinator compresses cheeks. Serratus anterior protracts scapula, not adducts. Sternocleidomastoid turns the head. Trapezius' dual role in scapular and neck movement sets it apart, key for posture and head positioning.

Correct Answer: D

Rationale: Iliopsoas flexes the thigh, lifting it toward the torso, as in stepping. Gluteus maximus extends it, straightening the hip, as in standing. Abduction-adduction involves lateral shifts, not their hip-centric actions. Flexion-extension captures their opposing roles, distinct from side movements, essential for thigh positioning.

Correct Answer: A

Rationale: Myasthenia gravis, an autoimmune condition, weakens muscles by antibodies destroying acetylcholine receptors, impairing nerve-muscle signals, causing fatigue. Amyotrophic lateral sclerosis (ALS, Lou Gehrig's) is neurodegenerative, not autoimmune. Fibromyalgia is pain-focused, not receptor-based. Myasthenia's receptor-specific weakness sets it apart, treatable with immunosuppressants, distinct from nerve or pain conditions.

Correct Answer: A

Rationale: The coronal suture runs transversely across the skull, separating the frontal bone (forehead) from the two parietal bones (top sides). This suture, visible as a line in coronal section, allows cranial growth in infancy and fuses later. The sagittal suture, midline, joins the parietal bones, not the frontal. The lambdoid suture separates parietals from the occipital bone posteriorly. 'Frontal' as a suture is incorrect it's the bone itself. The coronal suture's position and function distinguish it, essential for skull structure and development, unlike sagittal or lambdoid orientations or misnamed bone references.