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 nontender on examination, and the anion gap is normal.For the above patient with new symptoms, select the most likely acid base disorder.

**Core Concept**
The patient's presentation of sudden-onset non-bloody diarrhea suggests a gastrointestinal cause for her acid-base disorder. In this context, the normal anion gap indicates that the disorder is likely metabolic acidosis with a non-anion gap (or normal anion gap) cause.

**Why the Correct Answer is Right**
Metabolic acidosis with a normal anion gap is often caused by the loss of bicarbonate ions from the gastrointestinal tract. In this case, the sudden onset of non-bloody diarrhea suggests a secretory diarrhea, which leads to the loss of bicarbonate-rich intestinal secretions. This results in a decrease in the bicarbonate concentration in the blood, causing metabolic acidosis. The normal anion gap rules out other causes of metabolic acidosis, such as lactic acidosis or ketoacidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Metabolic alkalosis is incorrect because the patient's symptoms and normal anion gap suggest a loss of bicarbonate, not an excess of bicarbonate.
**Option B:** Respiratory acidosis is incorrect because the patient's symptoms and normal anion gap do not suggest any primary respiratory issue.
**Option C:** Mixed disorder (respiratory acidosis and metabolic acidosis) is incorrect because the patient's normal anion gap and absence of respiratory symptoms make this option unlikely.

**Clinical Pearl / High-Yield Fact**
In patients with diarrhea, the anion gap is often normal because the loss of bicarbonate-rich intestinal secretions is balanced by the loss of chloride ions. This can make it difficult to diagnose metabolic acidosis in these patients, and a high clinical suspicion is often necessary.

**Correct Answer:** D. Metabolic acidosis

✓ Correct Answer: A. metabolic acidosis