This type of muscle works by stabilizing the origin of the agonist so that it can act more efficiently.

Correct Answer: D

Rationale: Fixators stabilize the origin of the agonist (prime mover), ensuring its force is directed efficiently toward the intended movement. For example, during arm abduction, scapular fixators (e.g., trapezius) hold the scapula steady so the deltoid can lift the arm. Synergists assist the agonist by refining motion, not stabilizing origins. The agonist is the main mover, not a stabilizer. Antagonists oppose the agonist. Fixators' role in anchoring the origin enhances movement precision, making it the correct answer, as it supports the agonist's action by preventing unwanted shifts at the stationary attachment, a key aspect of coordinated muscle function.

Correct Answer: B

Rationale: Hip extension, critical for climbing steps, is primarily powered by the gluteus maximus, which extends the thigh backward. Weakness here impairs stair ascent, while intact hip flexion (iliopsoas) and knee flexion/extension (hamstrings/quadriceps) suggest a specific issue. Adductor magnus adducts, not extends. Gluteus medius abducts and stabilizes. Semitendinosus flexes the knee, not hip extension. Gluteus maximus is the correct answer, as its damage aligns with isolated hip extension weakness, a key mover in upward locomotion.

Correct Answer: C

Rationale: The body has three muscle types: cardiac, smooth, and skeletal, each with distinct roles. Cardiac muscle, in the heart, pumps blood involuntarily. Smooth muscle, in organs like the intestines, manages involuntary tasks like digestion. Skeletal muscle, attached to bones, drives voluntary movements like walking. Suggesting four or five types might confuse these with subtypes (e.g., fast-twitch skeletal) or non-muscle tissues, but anatomy recognizes only three based on structure and control. Two undercounts by omitting one, missing the full scope. Three aligns with histological and functional classification, reflecting their specialized purposes cardiac for circulation, smooth for visceral action, skeletal for locomotion. This trio covers all muscular functions, providing a clear, consistent framework for understanding the body's movement and maintenance systems.

Correct Answer: C

Rationale: Muscle types are cardiac, smooth, and skeletal, defined by structure and function. Cardiac drives the heart, smooth lines organs, and skeletal moves bones. Sesamoids are small bones embedded in tendons, like the patella, not muscles they enhance leverage, not contract. Including them as muscle misrepresents anatomy, as they're skeletal components. The three muscle types cover all contractile tissues, while sesamoids support mechanically, lacking muscle's cellular traits, making them the clear non-muscle outlier in this context.

Correct Answer: D

Rationale: Myoglobin, a red pigment in muscle fibers, binds and stores oxygen, releasing it during activity when blood supply can't meet demand, like in intense exercise. This supports aerobic respiration in mitochondria, especially in slow-twitch fibers. Hemoglobin, also red, transports oxygen in blood, not storing it in muscles. Erythrocytes are red blood cells carrying hemoglobin, not pigments themselves. Sarcoplasm is the muscle cell's cytoplasm, not a pigment or oxygen store. Myoglobin's muscle-specific oxygen storage distinguishes it, enhancing endurance by buffering oxygen availability, unlike hemoglobin's circulatory role or the non-storage nature of sarcoplasm and erythrocytes, aligning with its biochemical function in muscle tissue.