A 60-year-old man with chronic gastric outlet obstruction from cicatrizing duodenal ulcer has vomiting of large volumes of food residue. His biochemistry shows sodium 138 mEq/L, potassium 2.8 mEq/L, chloride 88 mEq/L, bicarbonate 38 mEq/L, and urine pH 5 despite systemic alkalaemia. What best explains the urinary findings?
- A Paradoxical aciduria due to severe volume depletion and aldosterone-driven sodium conservation with hydrogen ion excretion ✓
- B Renal tubular acidosis from chronic potassium depletion
- C Diuretic-induced metabolic alkalosis
- D Primary hyperaldosteronism with potassium wasting
Correct answer: A. Paradoxical aciduria due to severe volume depletion and aldosterone-driven sodium conservation with hydrogen ion excretion
Explanation
Gastric outlet obstruction produces hypochloraemic hypokalaemic metabolic alkalosis from loss of hydrochloric acid in vomitus. Early on the kidney excretes bicarbonate, but once severe volume depletion occurs, aldosterone-mediated sodium retention forces hydrogen ion excretion alongside potassium, producing acidic urine despite systemic alkalosis, termed paradoxical aciduria. Renal tubular acidosis would give alkaline urine, so it cannot explain the key.
Reference: Bailey and Love's Short Practice of Surgery, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.