This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fluid & Electrolyte, Subject name: Medicine
Normal anion gap is seen in all except 
Select and explain. 
A. Ureterosigmoidoscopy, B. Methanol poisoning, C. Renal tubular acidosis, D. Diarrhoea
B: Methanol poisoning
Answer: B: Methanol poisoning 
Explanation: Ans: b) Methanol poisoning (Ref: Harrison 17th edition/ Page 291-292)Anion gap is the measure of unmeasured anions.AG = Na+-Normally anions = cationsAny increase in anions other than Cl- or HCO3- will cause decrease in Cl- or HCO2 so that total anions is same in quantity. Eg increase in acetate, lactate, methanol, phosphate, albumin will increase AGDecrease in unmeasured cation (Ca++, Mg++ K+) will also have same result - increase AGExample of high anion gap acidosis:Lactic acidosis, Ketoacidosis, Ethylene glycol, Methanol, Salicylates, and Renal failure.Normal anion gap: 10-12 mmol/LNormal anion gap acidosis:Loss of HCO3- from GIT or kidney will result in reciprocal change in - No change in anion gap ( and vice versa)Eg : 1) RTA/ Ureterosigmoidoscpy/CaCl,2) Renal failure with drug induced hyperkalemia3) Cation exchange resins