In the classification of two organisms, which of the following is the best evidence of a close relationship?

Correct Answer: B

Rationale: Close evolutionary relationships are best shown by genetic similarity. Amino acid sequences in proteins, encoded by DNA, reflect shared ancestry highly conserved sequences indicate recent divergence, as with hemoglobin in mammals. Bone structure suggests relation but converges in unrelated species (e.g., bird and bat wings). Reproduction and respiration methods vary widely even among kin. Protein sequences' molecular precision, tied to genetic heritage, outweighs morphological or functional traits, making it the gold standard in phylogenetic analysis.

Correct Answer: D

Rationale: Myosin heads bind actin, change shape via ATP hydrolysis, and slide actin toward the sarcomere's center, the core of sliding filament theory, shortening the muscle. Actin doesn't slide myosin it's pulled. Troponin regulates, not slides, myosin or actin it shifts tropomyosin. Myosin sliding actin defines the power stroke, distinguishing it from regulatory or reversed actions, essential for contraction mechanics.

Correct Answer: A

Rationale: Calcium drives contraction by binding troponin, a thin filament protein. This binding alters troponin's shape, shifting tropomyosin away from actin's myosin-binding sites, exposing them for cross-bridge formation. Acetylcholine triggers the initial nerve signal, not calcium's role. Cross-bridge disengagement follows ATP binding, not calcium's action, and ATP hydrolysis energizes myosin beforehand. Calcium's specific effect on troponin unveils actin sites, enabling the sliding filament mechanism, a pivotal step distinguishing it from nerve initiation or energy cycles in muscle physiology.

Correct Answer: B

Rationale: Semimembranosus and semitendinosus, with biceps femoris, form the hamstrings on the posterior thigh, aiding knee flexion and hip extension. The seminal vesicle is a reproductive organ, the forearm contains arm flexors/extensors, and back muscles (e.g., latissimus dorsi) differ in function and location. The thigh's dorsal placement aligns with hamstring anatomy, distinguishing it from unrelated regions, key for understanding lower limb movement.

Correct Answer: A

Rationale: Deltoid reflects shape triangular, like the Greek delta not location, unlike extensor carpi ulnaris (wrist), rectus abdominus (abdomen), and biceps femoris (thigh's femur). Located on the shoulder, deltoid's name emphasizes form over position, distinguishing it from location-specific terms, useful for anatomical recognition beyond regional cues.