The best indicator of blood osmolarity is the blood concentration of ______ ions.

Correct Answer: D

Rationale: Sodium is the best osmolarity indicator dominant extracellular ion (~140 mM, e.g., drives osmosis). Calcium is minor (~2.5 mM) less impact. Chlorine follows Na secondary. Potassium is intracellular not plasma. Na 's prevalence distinguishes it, critical for osmolarity, unlike lesser ions.

Correct Answer: B

Rationale: Hemodialysis removes 8-25 g urea in 3-6 hours e.g., typical clearance (blood levels drop). 8-10 g is low underestimates. 50-250 g is excessive beyond capacity. 8-25 mg is trivial mg vs. g error. 8-25 g distinguishes it, critical for dialysis efficacy, unlike under/over or unit mistakes.

Correct Answer: C

Rationale: Distal tubules are impermeable to renin it's a hormone, not reabsorbed (e.g., acts systemically). Aldosterone affects Na cells respond. ADH adjusts water collecting duct permeable. Macula densa is a structure not a substance. Renin's impermeability distinguishes it, key to its systemic role, unlike hormones affecting tubules.

Correct Answer: B

Rationale: In alkalosis, kidneys excrete HCO₃⁻ less CO₂ binding allows loss (e.g., distal tubule adjustment). More Na reabsorption doesn't fix base excess unrelated. Angiotensin/ADH affects water not pH directly. K absorption is minor irrelevant. HCO₃⁻ excretion distinguishes it, critical for pH correction, unlike Na or K shifts.

Correct Answer: B

Rationale: Kidney innervation (sympathetic) regulates arteriole diameter e.g., constricts afferent/efferent, adjusts GFR. Filtrate composition is glomerular not neural. Water reabsorption is hormonal (ADH) not innervation. Brush border is structural not regulated. Arteriolar control distinguishes it, key to blood flow, unlike filtrate or tubular functions.