Tim ate something that is not agreeing with his digestive tract. He needs to go to the bathroom to defecate, but there is a line. What muscle helps keep the anal canal and anus closed?

Correct Answer: D

Rationale: The external anal sphincter, a voluntary skeletal muscle, keeps the anal canal and anus closed to prevent defecation until appropriate, allowing Tim to wait. Bulbospongiosus aids in urination and erection, not anal control. Ischiocavernosus supports erection, not defecation. Obturator internus rotates the thigh, unrelated here. The external anal sphincter is the correct answer, as it provides conscious control over defecation, distinct from the involuntary internal sphincter, and is key in maintaining continence under digestive distress.

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.

Correct Answer: A

Rationale: Muscle fatigue sets in when ATP depletes, halting cross-bridge cycling as myosin can't detach or re-cock without energy, despite calcium presence. This follows prolonged activity outpacing ATP regeneration. The latent period, pre-contraction, involves signal delay, not fatigue. Relaxation begins as calcium returns to the SR, not ATP exhaustion. Lactic acid breakdown isn't a fatigue marker it accumulates, not depletes, during anaerobic effort. ATP shortage directly impairs contraction, distinguishing fatigue from timing phases or metabolic byproducts, reflecting energy failure's impact on muscle performance.