A group of disorders characterized by progressive degeneration and weakening of muscles is

Correct Answer: D

Rationale: Muscular dystrophy encompasses genetic disorders like Duchenne, progressively weakening muscles via protein defects, leading to loss of function. Sprains are ligament injuries. Fibromyalgia is pain without degeneration. Tendonitis inflames tendons. Muscular dystrophy's degenerative nature distinguishes it, impacting muscle integrity over time.

Correct Answer: C

Rationale: ATP (adenosine triphosphate) is the immediate energy source for muscle contraction, directly fueling myosin cross-bridge cycling by hydrolyzing to ADP and phosphate, releasing energy to pull actin filaments. Glucose, a fuel, must be metabolized via glycolysis or respiration to produce ATP, not an instant source. ADP, a product of ATP breakdown, needs rephosphorylation to become usable ATP. AMP, further down the energy chain, isn't a primary energy carrier. ATP's ready availability in muscle cells ensures instant contraction energy, distinguishing it from substrates requiring processing or depleted molecules, critical for rapid muscle action.

Correct Answer: A

Rationale: The origin, a muscle's fixed attachment (e.g., biceps on scapula), moves least during contraction, anchoring the muscle as it pulls the insertion (e.g., radius) toward it. The insertion moves most, effecting motion. Ligaments, stabilizing joints, don't move but stretch minimally. Joints shift position via bone movement, more than the origin. The origin's stability provides leverage, distinguishing it as the least mobile, critical for force transmission, unlike the dynamic insertion, passive ligaments, or pivoting joints in muscle action.

Correct Answer: B

Rationale: The masseter, from zygomatic arch to mandible, elevates the jaw, closing it for chewing, a primary masticatory muscle. Swallowing involves pharyngeal muscles (e.g., constrictors). Lip movement uses orbicularis oris. Eye closure is orbicularis oculi's role. Masseter's powerful jaw elevation distinguishes it, key for biting and grinding, unlike swallowing, lip, or eye actions.

Correct Answer: B

Rationale: Gastrocnemius, spanning the calf, is the largest lower leg muscle, plantarflexing the foot with two heads. Soleus, beneath it, assists but is smaller. Tibialis anterior dorsiflexes, smaller and anterior. Tibialis posterior supports the arch, not largest. Gastrocnemius' bulk and power distinguish it, key for walking and jumping.