This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Excretory System (Kidney, Bladder) Acid-Base Balance, Subject name: Physiology
Normal anion gap metabolic acidosis is seen in: 
Select and explain. 
A. Diarrhoea, B. Renal failure, C. Lactic acidosis, D. Diabetic ketoacidosis
A: Diarrhoea
Answer: A: Diarrhoea 
Explanation: Law of electroneutrality = Number of cations & anions should be equal in ICF and ECF In ECF, Na+ is main cation, Cl- & HCO3- are main anions. We do not measure all cations & anions, it is measured as + + = + + we can measure all cations but we cannot measure all anions. So, there is a gap created b/w these two sides. This is called Anion gap. Anion gap is calculated as -{ + } = 141 - = 141 - 129 = 12meq/lit +-4(K+) Thus, normal anion gap is 12+- 4 meq/lit. In acidosis conditions; HCO3- decreases & thus anion gap increases except in acidosis conditions in which anion of the acid is Cl- where anion gap will remain normal. In G.I fluid loss, anion gap remains normal.