Urea concentration in kidney structures

Steps:

• Identify renal structures involved in urine formation: Bowman's capsule, proximal tubule, loop of Henle, collecting duct.
• Recall glomerular filtration produces fluid with urea concentration matching plasma (low, ~5 mM).
• Note urea concentration rises in tubules due to water reabsorption and urea secretion.
• Compare: initial filtrate has lowest urea; final urine has highest.

Why A is correct:

• Bowman's capsule contains glomerular filtrate, which has low urea concentration identical to plasma per filtration definition.

Why the others are wrong:

• B (proximal tubule): Urea concentration increases slightly due to water reabsorption.
• C (loop of Henle): Urea enters in ascending limb, raising concentration via countercurrent mechanism.
• D (collecting duct): High urea from ADH-mediated water reabsorption and urea recycling.

Final answer: A