Urinary anion gap is an indication of excretion of which of the following?

Urinary anion gap is calculated using the equation, urinary anion gap = ( Na+ + K+) - (Cl-). It reflects the ability of the kidney to excrete NH4Cl. During metabolic acidosis, kidneys normally eliminate NH4+Cl- to enhance the removal of excess H+ion. In response to acidosis, daily urinary excretion of NH4Cl can be increased from 30 mEq to 200 mEq. Ref: Harrison's Principle of Internal Medicine, 16th Edition, Page 368 ; Current Medical Diagnosis and Treatment 2012, Chapter 21