Which motions can be made by the upper limbs but NOT by the lower limbs?

Correct Answer: A

Rationale: Pronation (palm down) and supination (palm up) are unique to the upper limbs, enabled by the radius rotating over the ulna, a feature absent in the lower limbs due to fixed tibia-fibula alignment. Flexion/extension (e.g., knee bending) and adduction/abduction (e.g., leg spreading) occur in both upper and lower limbs. Lateral/medial rotation happens at the hip and shoulder, not distinguishing them. Pronation and supination are the correct answer, as they're exclusive to forearm anatomy, reflecting a key functional difference between upper and lower limb mobility critical for tasks like turning a doorknob.

Correct Answer: C

Rationale: Ligaments are tough, elastic bands connecting bone to bone, stabilizing joints like the knee's ACL linking femur to tibia. This limits excessive movement, maintaining skeletal integrity. Cartilage-to-bone connections occur via other tissues, not ligaments directly, as cartilage cushions, not links. Muscle-to-bone attachment is tendons' job, pulling bones for motion. Tendons-to-muscles isn't accurate tendons extend from muscle to bone, not connecting to other tendons. Ligaments' bone-to-bone role distinguishes them, passively restricting joint range while allowing flexibility, unlike tendons' active force transmission or cartilage's shock absorption. This structural specificity ensures joint stability across the body, vital for posture and movement, setting ligaments apart in the musculoskeletal framework.

Correct Answer: D

Rationale: The muscular system accounts for about 50% of body weight, primarily from skeletal muscles, with smooth and cardiac adding less mass. This reflects their extensive presence over 600 muscles supporting movement and structure. Twenty percent is too low, ignoring skeletal bulk. Forty percent is close but underestimates, per anatomical data. Seventy percent overstates, leaving little for bones, organs, and fat. Fifty percent aligns with standard physiology, emphasizing muscles' role in force generation and mass, balancing the body's composition, distinct from lower or higher guesses misaligned with muscle distribution.

Correct Answer: C

Rationale: Calcium ions, essential for muscle contraction, are stored in the sarcoplasmic reticulum, a specialized endoplasmic reticulum surrounding myofibrils. Upon a nerve signal, it releases calcium into the sarcoplasm to bind with troponin, initiating actin-myosin interaction. T-tubules conduct the signal inward but don't store calcium they facilitate its release. The sarcolemma is the muscle fiber's membrane, encasing the cell, not a storage site. Mitochondria produce ATP, powering contraction, but lack calcium storage capacity. The sarcoplasmic reticulum's role as a calcium reservoir is unique, ensuring precise regulation of contraction and relaxation, distinguishing it from conduction, structural, or energy-producing components, critical for the sliding filament mechanism in muscle physiology.

Correct Answer: C

Rationale: After ATP depletion in muscle contraction, creatine phosphate rapidly donates a phosphate to ADP, regenerating ATP in seconds via creatine kinase. This immediate source sustains short bursts of activity. Cellular respiration, in mitochondria, follows later, using oxygen for sustained ATP but takes time. Lactic acid breakdown isn't a process lactic acid forms in anaerobic conditions, not regenerating ATP. Fermentation (anaerobic glycolysis) produces ATP slowly, secondary to creatine phosphate. Its quick, oxygen-independent action distinguishes creatine phosphate, bridging energy gaps, unlike slower respiration or fermentation, vital for initial muscle energy demands.