What is the source of the majority of the energy needed by muscles for physical activity that continues for longer than 30 or 40 minutes?

Correct Answer: D

Rationale: Sustained activity beyond 30-40 minutes relies on aerobic respiration in mitochondria, oxidizing pyruvate (from glycolysis) with oxygen to produce 36-38 ATP per glucose far more than stored ATP (seconds), glycolysis (minutes), or creatine phosphate (15 seconds). This efficient, oxygen-dependent process supports endurance, distinguishing it from short-burst energy sources, critical for prolonged muscle function in activities like running.

Correct Answer: D

Rationale: The flexor carpi ulnaris flexes the wrist, bending it toward the forearm's palmar side, mirroring the flexor carpi radialis' action, differing only in position (ulnar vs. radial). It doesn't flex the arm or fingers, nor extend like its extensor counterpart. This wrist-specific flexion distinguishes it, key to hand positioning movements.

Correct Answer: D

Rationale: Acetylcholine, released by somatic motor neurons, crosses the neuromuscular junction, binding sarcolemma receptors to depolarize the membrane, triggering an action potential. Inorganic phosphate and ADP relate to ATP cycling, calcium regulates contraction internally not membrane depolarization. Acetylcholine's neurotransmitter role initiates muscle activation, distinguishing it from intracellular or energy-related molecules, key to voluntary movement.

Correct Answer: C

Rationale: Cardiac muscle is striated but involuntary (autonomic), not voluntary unlike skeletal (striated, voluntary, syncytial) and smooth (visceral, involuntary, non-striated). This mischaracterization distinguishes cardiac's automatic heart function, critical for physiological accuracy, contrasting with skeletal's conscious control.

Correct Answer: C

Rationale: Gluteus maximus is avoided for injections due to sciatic nerve proximity; gluteus medius, deltoid (shoulder), and vastus lateralis (thigh) are safer, with ample muscle mass. This safety distinction marks gluteus maximus as non-standard, critical for clinical practice.