**Core Concept**
Metabolic acidosis is a clinical condition characterized by a decrease in blood pH due to an increase in the concentration of acidic substances in the blood, such as lactic acid, ketoacids, or other organic anions. It occurs when the body produces more acid than it can eliminate, leading to a net increase in acid in the blood.

**Why the Correct Answer is Right**
The correct answer is D. Lactic acidosis is a specific type of metabolic acidosis caused by the accumulation of lactate in the blood due to impaired lactate metabolism. This can occur in various conditions such as type B lactic acidosis due to mitochondrial dysfunction, diabetic ketoacidosis, or other metabolic disorders. The accumulation of lactate leads to an increase in the anion gap, which is a key feature of metabolic acidosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it refers to a condition that actually causes metabolic alkalosis, not acidosis. Metabolic alkalosis is a condition characterized by an increase in blood pH due to a decrease in the concentration of acidic substances in the blood.

**Option B:** This option is incorrect as it refers to a condition that can cause respiratory acidosis, not metabolic acidosis. Respiratory acidosis is a condition characterized by an increase in blood CO2 levels, leading to a decrease in blood pH.

**Option C:** This option is incorrect as it refers to a condition that can cause metabolic alkalosis, not acidosis. Metabolic alkalosis is a condition characterized by an increase in blood pH due to a decrease in the concentration of acidic substances in the blood.

**Clinical Pearl / High-Yield Fact**
In metabolic acidosis, the anion gap is increased due to the accumulation of unmeasured anions such as lactate, ketoacids, or other organic anions. The anion gap is calculated as the difference between the sodium and chloride concentrations in the blood, and an increase in the anion gap is a key feature of metabolic acidosis.

**Correct Answer: D. Lactic acidosis**