**Core Concept**
The plasma anion gap is a calculated difference between the concentrations of the major cations (positively charged ions) and anions (negatively charged ions) in the blood, providing insight into the underlying cause of metabolic acidosis.

**Why the Correct Answer is Right**
The plasma anion gap is calculated using the formula: Anion Gap = (Na+ + K+) - (Cl- + HCO3-). In this case, the values are Na+ = 140 mEq/L, K+ = 3 mEq/L, Cl- = 112 mEq/L, and HCO3- = 16 mEq/L. Substituting these values into the formula, we get: Anion Gap = (140 + 3) - (112 + 16) = 143 - 128 = 15 mEq/L. This indicates a normal anion gap metabolic acidosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not provide a valid calculation for the anion gap.
* **Option B:** This option is incorrect because it would represent a high anion gap metabolic acidosis, typically seen in conditions such as diabetic ketoacidosis or lactic acidosis.
* **Option C:** This option is incorrect because it would represent a normal anion gap metabolic acidosis, but the calculation would be incorrect.

**Clinical Pearl / High-Yield Fact**
A normal anion gap metabolic acidosis is often seen in conditions such as gastrointestinal bicarbonate loss (e.g., diarrhea), renal tubular acidosis, or excessive chloride ingestion.

**Correct Answer: D. 15**