**Core Concept**
A widened anion gap is a laboratory finding that occurs when the difference between the measured cations (sodium and potassium) and anions (chloride and bicarbonate) in the blood exceeds 10-12 mmol/L. This imbalance is typically caused by the accumulation of unmeasured anions, such as ketones, lactate, or other organic acids.

**Why the Correct Answer is Right**
Diarrhea primarily causes a loss of bicarbonate ions (HCO3-) through the gastrointestinal tract, which can lead to a metabolic acidosis but does not typically result in a widened anion gap. This is because the lost bicarbonate ions are replaced by chloride ions (Cl-), which are already measured in the anion gap calculation. In contrast, conditions like lactic acidosis (A), diabetic keto-acidosis (C), and methanol poisoning (D) all lead to the accumulation of unmeasured anions, such as lactate, ketones, and formic acid, respectively, which increase the anion gap.

**Why Each Wrong Option is Incorrect**
**Option A:** Lactic acidosis is a classic cause of a widened anion gap, as lactate is an unmeasured anion that accumulates in the setting of tissue hypoxia or impaired pyruvate metabolism.
**Option C:** Diabetic keto-acidosis is another common cause of a widened anion gap, as the accumulation of ketones (acetoacetate and beta-hydroxybutyrate) leads to an increase in unmeasured anions.
**Option D:** Methanol poisoning results in the formation of formic acid, an unmeasured anion that contributes to a widened anion gap.

**Clinical Pearl / High-Yield Fact**
In cases of suspected anion gap acidosis, it is essential to consider the underlying cause and potential contributing factors, such as tissue hypoxia, sepsis, or medication toxicity. A thorough history, physical examination, and laboratory evaluation are crucial for guiding treatment and preventing complications.

**✓ Correct Answer: B. Diarrhea**