What is the middle layer of the kidney?

Correct Answer: C

Rationale: The kidney's layered anatomy includes the cortex as the outer region, the renal pelvis as the innermost collecting area, and the renal medulla as the intermediate zone. The medulla, situated between the cortex and pelvis, contains renal pyramids triangular structures that channel urine toward the pelvis via collecting ducts. Unlike the cortex, which filters blood, the medulla concentrates urine, regulating water and electrolyte balance through its loop of Henle and collecting tubules. The nephron is a functional unit crossing multiple layers, not a layer itself, while the renal pelvis is a central cavity. The medulla's middle position and its role in urine concentration distinguish it, reflecting the kidney's progressive processing from filtration to excretion.

Correct Answer: C

Rationale: A Foley catheter, a flexible tube inserted through the urethra into the bladder, primarily drains urine in patients unable to urinate naturally, such as post-surgery or with urinary retention. Its balloon tip secures it, allowing continuous drainage into a bag. It doesn't administer fluids or medications those use IVs or other catheters nor monitor pressure, which requires sensors. Draining urine, its core medical function, prevents complications like infection or kidney damage, distinguishing it in clinical practice.

Correct Answer: B

Rationale: Muscle structure descends from large to small: myofibrils (organelles within fibers) contain sarcomeres (contractile units), which comprise thin (actin) and thick (myosin) filaments. Muscle belly, the whole muscle, is larger, so starting there misorders scale. Sarcomeres, between Z-lines, house filaments, not vice versa filaments build sarcomeres, sarcomeres build myofibrils. Starting with filaments then sarcomere reverses this hierarchy, and myofibril as largest ignores its cellular role. The correct sequence reflects anatomical organization, distinguishing myofibril as a bundle of sarcomeres, each with filaments, key to understanding contraction's structural basis.

Correct Answer: D

Rationale: Myosin heads stay bound to actin post-power stroke until another ATP binds, detaching myosin by altering its conformation, allowing the cycle to repeat or relax. More Ca²⁺ affects troponin, not detachment Ca²⁺ sustains contraction. Troponin doesn't bind actin directly for release. More acetylcholine restarts excitation, not detachment. ATP's binding to myosin triggers release, distinguishing it from Ca²⁺ or neurotransmitter roles, key to cross-bridge cycling.

Correct Answer: B

Rationale: Muscle cells hierarchically organize contractile units. Myofibrils, spanning the cell's length, are bundles of sarcomeres repeating units with thick (myosin) and thin (actin) myofilaments. Sarcomeres, about 2 micrometres long, link end-to-end within myofibrils, which extend tens to hundreds of micrometres. Myofilaments are shorter segments within sarcomeres, and troponin is a tiny protein on actin. Myofibrils' full-cell length distinguishes them as the longest, critical for coordinated contraction across the fibre, unlike shorter sarcomeres or molecular-scale components.