**Core Concept**
High anion gap metabolic acidosis occurs when there is an increase in the concentration of unmeasured anions in the blood, leading to a discrepancy between the measured and calculated anion gaps. This condition is typically caused by the accumulation of acidic substances, such as lactic acid, ketoacids, or toxic ingestions.

**Why the Correct Answer is Right**
The correct answer is a condition that does not cause an increase in the anion gap. Normal anion gap metabolic acidosis, also known as hyperchloremic metabolic acidosis, occurs when there is a loss of bicarbonate or an addition of chloride ions, without an increase in unmeasured anions. This type of acidosis is commonly seen in gastrointestinal bicarbonate loss, such as in diarrhea, or in the use of certain medications like acetazolamide.

**Why Each Wrong Option is Incorrect**
**Option A:** Lactic acidosis is a classic cause of high anion gap metabolic acidosis, typically seen in conditions of tissue hypoxia, such as shock or severe sepsis.
**Option B:** Methanol and ethylene glycol ingestions also cause high anion gap metabolic acidosis by the accumulation of their metabolites, formic acid and oxalic acid, respectively.
**Option C:** Diabetic ketoacidosis is another common cause of high anion gap metabolic acidosis, resulting from the accumulation of ketoacids produced by the breakdown of fatty acids.

**Clinical Pearl / High-Yield Fact**
Remember that a high anion gap is a hallmark of metabolic acidosis caused by lactic acidosis, ketoacidosis, or toxic ingestions, whereas normal anion gap acidosis is typically seen in gastrointestinal bicarbonate loss or medication-induced hyperchloremic acidosis.

**Correct Answer: D. Normal anion gap metabolic acidosis**