A 25-year-old man with recurrent nephrolithiasis has serum bicarbonate of 19 mEq/L, serum potassium 3.0 mEq/L, pH 7.32, urine pH consistently 6.5 (inappropriately alkaline), and nephrocalcinosis on ultrasound. The underlying defect is:
- A Distal (type 1) renal tubular acidosis — failure to secrete H+ in collecting duct ✓
- B Proximal (type 2) RTA — bicarbonate wasting in proximal tubule
- C Type 4 RTA — hyperkalaemic hyperchloraemic acidosis from aldosterone deficiency
- D Anion gap metabolic acidosis from lactic acidosis
Correct answer: A. Distal (type 1) renal tubular acidosis — failure to secrete H+ in collecting duct
Explanation
Distal RTA (type 1) is characterised by inability of the collecting duct to secrete H+, resulting in a persistently alkaline urine pH (>5.5) even in the face of systemic acidosis. This leads to hypokalaemia (via secondary aldosteronism), nephrocalcinosis, and recurrent calcium phosphate stones. Type 2 RTA shows very low urine pH during acidosis (<5.5) when the filtered bicarbonate load falls below the tubular threshold. Type 4 RTA causes hyperkalaemia, not hypokalaemia.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.