- anion gap = Na - Cl and HCO3
- metabolic acidosis means low bicarb
Therefore, any normal anion gap metabolic acidosis is essentially hyperchloremic metabolic acidosis. See normal anion gap mnemonic thread
In proximal renal tubular acidosis you have failure of bicarbonate reabsorption and this will be compensated by chloride reabsorption to maintain neutrality across the renal tubule membrane. See this thread for a similar issue