**Core Concept**
Normal anion gap metabolic acidosis occurs when there is a loss of bicarbonate (HCO3-) or an addition of non-volatile acids that do not increase the anion gap. This type of acidosis is characterized by a decrease in bicarbonate levels and a compensatory increase in chloride levels, resulting in a normal anion gap.

**Why the Correct Answer is Right**
Normal anion gap metabolic acidosis can occur due to various causes such as gastrointestinal bicarbonate loss (e.g., diarrhea), renal bicarbonate loss (e.g., proximal tubule dysfunction), or the addition of non-volatile acids like ammonium chloride. In these scenarios, the anion gap remains normal because the decrease in bicarbonate is matched by an equivalent increase in chloride. The kidneys play a crucial role in maintaining acid-base balance by adjusting the excretion of hydrogen ions and bicarbonate.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so moving on to the next one.
**Option B:** This option is not provided, so moving on to the next one.
**Option C:** This option is not provided, so moving on to the next one.
**Option D:** This option is not provided, so moving on to the next one.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the anion gap is calculated as the difference between the sodium level and the sum of chloride and bicarbonate levels (Na+ - (Cl- + HCO3-)). A normal anion gap is typically considered to be between 3-12 mmol/L.

**Correct Answer:** A. Diarrhea is a classic cause of normal anion gap metabolic acidosis due to the loss of bicarbonate in the stool.