A 1 year old child presents with crossed eyes. While she seems fine and is able to recognize people, the patient is diagnosed with

Correct Answer: B

Rationale: Strabismus is the condition of misaligned eyes (crossed eyes), where eye muscles fail to coordinate, common in young children and treatable with therapy or surgery. The child recognizing people suggests vision is intact, just misaligned. Nystagmus is involuntary eye movement, not crossing. Presbyopia is age-related focus loss, irrelevant here. Myopia is nearsightedness, not alignment-related. Strabismus matches the crossed-eye presentation, making it the correct answer, as it's a muscle coordination issue distinct from refractive or movement disorders, often seen in pediatric cases.

Correct Answer: C

Rationale: The human body contains approximately 640 muscles, a widely accepted estimate in anatomy reflecting skeletal muscles primarily, which vary slightly by individual due to small accessory muscles. This number excludes smooth and cardiac muscles' microscopic units but focuses on distinct, named skeletal muscles enabling movement. Fewer, like 340, undercounts the extensive network across limbs, torso, and face. Higher figures like 860 might exaggerate by including minor variants or non-skeletal types, but 640 aligns with standard texts. It accounts for paired muscles (e.g., biceps) and smaller ones (e.g., in the ear), balancing precision with practicality. This figure underscores the muscular system's complexity, supporting diverse functions from walking to facial expressions, and provides a reliable benchmark for anatomical study, distinguishing it from overly broad or narrow estimates.

Correct Answer: C

Rationale: Cellular respiration encompasses breaking down food (glucose) with oxygen to produce energy (ATP) in cells, mainly via aerobic means in mitochondria, releasing carbon dioxide and water. Anaerobic respiration skips oxygen, yielding less energy and lactic acid. Aerobic respiration is a subset, but cellular respiration covers all energy-making processes, fitting broadly. Oxygen is a reactant, not the process. This oxygen-dependent breakdown fuels most organisms, contrasting anaerobic's limited scope, and its cellular scope includes glycolysis and beyond, making it the precise term for energy formation across living systems.

Correct Answer: C

Rationale: In striated muscles (skeletal and cardiac), the sarcomere is the functional contractile unit, spanning from one Z line to the next. It contains actin and myosin filaments that slide during contraction, shortening the sarcomere to produce force, as per the sliding filament theory. The Z band anchors actin but isn't the unit itself. Cross bridges are myosin heads interacting with actin, a mechanism within the sarcomere, not the unit. A myofibril is a larger structure of many sarcomeres. The sarcomere's role as the basic repeating segment driving contraction makes it the best answer, central to understanding muscle mechanics and striation patterns.

Correct Answer: D

Rationale: When ADP and inorganic phosphate (P) release from myosin's head after ATP hydrolysis, the cross-bridge bends, pulling actin toward the sarcomere's center the power stroke. This single event encompasses bridge bending, filament pulling, and the stroke itself, driving contraction. Each aspect isn't separate; they're facets of one action. Bending alone omits the pull, pulling alone skips mechanics, and power-stroke alone undersells the full process. All occur simultaneously as myosin resets, distinguishing this step from prior ATP binding or later detachment, critical for the sliding filament cycle and muscle shortening.