The only part in nephron which is permeable to water and impermeable to ions:

Correct Answer: B

Rationale: The thin descending limb is water-permeable, ion-impermeable osmosis concentrates filtrate (e.g., countercurrent). PCT is leaky ions/water pass. Cortical collecting duct adjusts with ADH ions move. Thick ascending pumps ions water-impermeable. Unique permeability distinguishes thin descending, critical for gradient, unlike leaky or adjustable segments.

Correct Answer: B

Rationale: Low [H₂CO₃] is difficult to measure rapidly dissociates to CO₂/H₂O (e.g., lab challenge). Easy estimation misstates requires CO₂ proxy. Ineffective ignores Henderson-Hasselbalch false. Replacement isn't needed CO₂ suffices. Measurement difficulty distinguishes it, critical for pH calculation, unlike estimation or equation errors.

Correct Answer: A

Rationale: The urinary system excretes waste (e.g., urea) and balances fluids/electrolytes not enzyme production (digestive), oxygen transport (respiratory), or temperature (skin). This defines its primary role, critical for homeostasis, contrasting with unrelated systems.

Correct Answer: B

Rationale: ADH increases water reabsorption in collecting ducts not aldosterone (Na+/K+), insulin (glucose), or oxytocin (labor). This identifies water regulation, key for urine concentration, contrasting with electrolyte or metabolic hormones.

Correct Answer: B

Rationale: Glycosuria is glucose in urine not proteinuria (protein), hematuria (blood), or pyuria (pus). This identifies diabetic marker, key for diagnosis, contrasting with other abnormalities.