## Core Concept
Metabolic acidosis is a condition characterized by an excessive amount of acid in the body fluids, often caused by an increase in acid production, a decrease in acid excretion, or a loss of bicarbonate. Certain medications can induce metabolic acidosis as a side effect by various mechanisms.

## Why the Correct Answer is Right
The correct answer, **D. Metformin**, is associated with the development of lactic acidosis, a form of metabolic acidosis. Metformin, a biguanide used in the treatment of type 2 diabetes, can cause lactic acidosis by inhibiting mitochondrial complex I, which leads to an increase in anaerobic metabolism and lactate production. This condition is rare but serious and can be fatal.

## Why Each Wrong Option is Incorrect
- **Option A:** Although certain antibiotics can cause side effects like gastrointestinal disturbances that might lead to metabolic acidosis indirectly, they are not commonly associated with causing metabolic acidosis directly.
- **Option B:** While some antiepileptic drugs can have various metabolic side effects, they are not commonly known for causing metabolic acidosis directly.
- **Option C:** This option does not provide a specific drug but mentioning a class like "antihistamines" can be misleading as they are not commonly associated with metabolic acidosis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that metformin is a common cause of lactic acidosis, particularly in patients with risk factors such as renal impairment, heart failure, or sepsis. Early recognition and discontinuation of metformin, along with supportive care, are crucial in managing metformin-induced lactic acidosis.

## Correct Answer: D. Metformin