This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Disorders of Sodium and Potassium, Subject name: Pediatrics
An 11 year child with failure to thrive, metabolic acidosis with increased anion gap. Not true regarding the differential diagnosis: 
Select and explain. 
A. Insulin dependent diabetes mellitus, B. Chronic renal failure, C. Renal tubular acidosis, D. Inborn errors of metabolism
C: Renal tubular acidosis
Answer: C: Renal tubular acidosis 
Explanation: Renal tubular acidosis - normal anion gap acidosis