What is the most common underlying condition leading to non-gap metabolic acidosis?

Non-gap metabolic acidosis occurs when there is a loss of bicarbonate or an increase in hydrogen ion concentration without the presence of unmeasured anions that would contribute to a high anion gap. Among the provided options, diarrhea is the most common underlying condition leading to this type of acidosis.

The loss of bicarbonate through the gastrointestinal tract is significant during diarrhea, as bicarbonate is secreted in the intestines to neutralize acids. This results in an increase in hydrogen ions and a corresponding decrease in bicarbonate, manifesting as a non-gap metabolic acidosis. The acidosis in this scenario is not due to an accumulation of acids or unmeasured anions, thus maintaining a normal anion gap.

Other conditions like chronic kidney disease and severe dehydration can contribute to metabolic acidosis as well. However, these tend to lead to high anion gap metabolic acidosis or are less common than diarrhea in the context of straightforward, non-gap acidosis. In the case of diabetes mellitus, although it can contribute to metabolic acidosis, it is more commonly associated with diabetic ketoacidosis, which presents with a high anion gap due to the accumulation of ketone bodies.

Thus, diarrhea stands out as the primary cause of non-gap metabolic