Renal and acid-base: proximal renal tubular acidosis: practice question

Try it

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Reduced proximal bicarbonate reabsorption, with intact distal acidification so urine can still become acidic once filtered bicarbonate is low

In proximal (type 2) renal tubular acidosis the proximal tubule fails to reabsorb the filtered bicarbonate, often as part of generalised Fanconi syndrome. Bicarbonate is lost until the plasma level falls enough for the filtered load to be handled by the distal nephron, which can still acidify urine to below 5.5. In distal (type 1) RTA the urine pH stays above 5.5 despite acidosis.

Option by option

• A. Failure of hydrogen ion secretion by distal alpha-intercalated cells, so urine pH stays above 5.5

  This is type 1 distal RTA, in which urine pH stays above 5.5.

• B. Reduced proximal bicarbonate reabsorption, with intact distal acidification so urine can still become acidic once filtered bicarbonate is low (correct)

  Correct. A proximal bicarbonate leak with preserved distal acidification gives acid urine at low serum bicarbonate.

• C. Hypoaldosteronism with hyperkalaemia and reduced ammonium excretion

  This describes type 4 RTA, which does not feature glycosuria and phosphaturia.

• D. Excess bicarbonate loss from the gut with an appropriate renal acid load

  Gut loss would not explain the proximal tubular wasting of glucose, phosphate and amino acids.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Physiology questions