**Core Concept**
Normal anion gap metabolic acidosis is a condition characterized by a decrease in serum bicarbonate levels, an increase in chloride levels, and a normal anion gap. This type of metabolic acidosis occurs when there is a loss of bicarbonate or an addition of acidic substances that do not increase the anion gap.

**Why the Correct Answer is Right**
Normal anion gap metabolic acidosis is typically caused by the loss of bicarbonate through the gastrointestinal tract or kidneys, or by the addition of acidic substances such as ammonium chloride or acetic acid. In these cases, the chloride levels increase to balance the loss of bicarbonate, resulting in a normal anion gap. The kidneys can also compensate for the loss of bicarbonate by reabsorbing more chloride ions.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided.
**Option B:** This option is not provided.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that a normal anion gap metabolic acidosis can be caused by various factors, including gastrointestinal losses, renal losses, or the addition of acidic substances. The anion gap is a useful tool in differentiating between high anion gap and normal anion gap metabolic acidosis.

**Correct Answer:** D.