Clinical Scenario:
A 12-year-old boy with chronic kidney disease due to reflux nephropathy is reviewed for persistent hyperkalaemia. He is clinically well and has no history of diarrhoea or vomiting. His medications include enalapril for hypertension.
Investigations show:
- Na⁺: 136 mmol/L
- K⁺: 6.0 mmol/L
- Cl⁻: 112 mmol/L
- HCO₃⁻: 17 mmol/L
- Creatinine: 1.7 mg/dL
- Glucose: 91 mg/dL
- Urine pH: 5.1
The metabolic acidosis persists despite correction of volume status. Which of the following best explains the development of metabolic acidosis in this patient?
A. Failure of distal H⁺ secretion
B. Reduced proximal bicarbonate reabsorption
C. Reduced renal ammonium excretion secondary to impaired aldosterone activity
D. Increased gastrointestinal bicarbonate loss
E. Increased urinary bicarbonate excretion due to distal tubular dysfunction
Correct answer & Explanation:
Correct answer: C. Reduced renal ammonium excretion secondary to impaired aldosterone activity
Explanation
The child has a normal-anion-gap metabolic acidosis with significant hyperkalaemia:
Anion gap = 136 − (112 + 17) = 7 mmol/L
The combination strongly suggests type 4 RTA (hyperkalaemic RTA).
Type 4 RTA results from reduced aldosterone activity or impaired aldosterone responsiveness, leading to reduced distal potassium secretion and hyperkalaemia. The hyperkalaemia itself further suppresses proximal tubular ammonium (NH₄⁺) production, reducing the amount of ammonia available to buffer secreted H⁺ in the collecting duct. This decreases net acid excretion, producing metabolic acidosis.
The urine pH of 5.1 is an important discriminator. Unlike distal/type 1 RTA, the kidney in type 4 RTA generally retains the ability to lower urine pH appropriately. The problem is predominantly reduced net acid excretion due to reduced ammonium availability, rather than an inability to secrete H⁺.
Enalapril can further reduce aldosterone activity and therefore aggravate hyperkalaemia and type 4 RTA.
Why the other options are incorrect
A. Failure of distal H⁺ secretion — Incorrect
This is characteristic of type 1 distal RTA, in which urine remains inappropriately alkaline despite systemic acidosis.
B. Reduced proximal bicarbonate reabsorption — Incorrect
This describes type 2 proximal RTA. It is generally associated with hypokalaemia rather than the prominent hyperkalaemia seen here.
D. Increased gastrointestinal bicarbonate loss — Incorrect
Diarrhoea causes hyperchloremic metabolic acidosis, but it does not explain persistent hyperkalaemia in this setting.
E. Increased urinary bicarbonate excretion due to distal tubular dysfunction — Incorrect
Bicarbonate wasting is primarily a feature of proximal RTA, not the fundamental abnormality in type 4 RTA.
High-yield CPSP distinction
| Type | Potassium | Urine pH during acidosis | Main defect |
|---|---|---|---|
| Type 1 | ↓ | >5.5 | Impaired distal H⁺ secretion |
| Type 2 | ↓ | Can fall <5.5 | Impaired proximal HCO₃⁻ reabsorption |
| Type 4 | ↑ | Usually <5.5 | Reduced aldosterone effect → ↓ NH₄⁺ excretion |
Exam clue:
Hyperkalaemia + normal-anion-gap metabolic acidosis + ability to acidify urine = think type 4 RTA.
References
- Palmer BF, Clegg DJ. Hyperkalemia across the continuum of kidney function. Clinical Journal of the American Society of Nephrology. 2018.
- Kher KK, Schnaper HW, Greenbaum LA. Clinical Pediatric Nephrology. Pediatric reference for renal tubular disorders and acid-base abnormalities.
- National Kidney Foundation. Clinical resources on hyperkalaemia and renal tubular disorders.
