This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Urinary tract, Subject name: Pathology
Which one of the following is not a feature of type I renal tubular acidosis? 
Select and explain. 
A. Lower tubule re-absorption of calcium, B. Defective bicarbonate absorption in distal tubule, C. Low serum potassium level, D. Increased anion gap
D: Increased anion gap
Answer: D: Increased anion gap 
Explanation: Mutations in CA2 and TCIRG1 result in osteopetrosis (in which bones are hard but brittle) and renal tubular acidosis. Ref: Robbins basic pathology 9th edition pg no: 801 Nephrocalcinosis is a diffuse deposition of calcium salts in renal tissue in a number of renal diseases, in hypercalcemia, hyperphosphataemia and renal tubular acidosis Ref: Textbook of pathology Harsh Mohan 6th edition page 684.