Q1: Answer: A
Rationale
Vibrio cholerae is a cause of secretory diarrhea. Fecal osmotic gap calculation can be useful in the evaluation of a patient with chronic diarrhea to differentiate between osmotic and secretory diarrhea. It is calculated by subtracting measured fecal electrolytes from normal lumen osmolality (290 – 2(fecal Na+ fecal K)). Secretory diarrhea is characterized by a fecal osmotic gap of less than 50 mOsm/kg, therefore answer A is the correct answer. Osmotic diarrhea is characterized by a fecal osmotic gap greater than 50 mOsm/kg or more specifically, more than 100 mOsm/kg, therefore answer B is incorrect. Accurate fecal osmotic gap calculation depends on normal stool osmolality, which is 290 mOsm/kg. Hypertonic (answer D) or dilute (answer C) stool samples are not appropriate for the calculation of the fecal osmotic gap, therefore answer C and D are incorrect.
