This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney, Subject name: Medicine
A 19-year-old girl develops sudden-onset non-bloody diarrhea. She was previously well and is not taking any medications or traveled anywhere recently. Her abdomen is soft and non-tender on examination, and the anion gap is normal.For the above patient with new symptoms, select the most likely acid base disorder. 
Select and explain. 
A. metabolic acidosis, B. metabolic alkalosis, C. respiratory acidosis, D. respiratory alkalosis
A: metabolic acidosis
Answer: A: metabolic acidosis 
Explanation: She has a non-anion gap metabolic acidosis because of bicarbonate loss from the diarrhea. The anion gap is calculated as the sodium concentration minus the chloride plus the bicarbonate concentration. Other causes of bicarbonate loss with normal anion gap include proximal RTA and primary hyperparathyroidism.