**Core Concept**
Metabolic acidosis with an increased anion gap in a child can be caused by various underlying conditions, including inborn errors of metabolism, toxic ingestions, and other disorders that lead to the accumulation of acidic metabolites or the loss of bicarbonate.

**Why the Correct Answer is Right**
The differential diagnosis for this presentation includes disorders such as methylmalonic acidemia, propionic acidemia, and other organic acidemias, which are caused by defects in the breakdown of certain amino acids or fatty acids. These conditions lead to the accumulation of acidic metabolites, resulting in metabolic acidosis with an increased anion gap. Additionally, conditions such as diabetic ketoacidosis (DKA) and lactic acidosis can also cause an increased anion gap metabolic acidosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** While diabetic ketoacidosis (DKA) can cause metabolic acidosis, it typically presents with a normal or low anion gap, not an increased anion gap.
* **Option B:** Lactic acidosis is a correct consideration in the differential diagnosis for metabolic acidosis with an increased anion gap.
* **Option C:** Renal tubular acidosis (RTA) can cause metabolic acidosis, but it typically presents with a normal anion gap, not an increased anion gap.
* **Option D:** This option is not provided, so we will skip it.

**Clinical Pearl / High-Yield Fact**
When evaluating a child with failure to thrive and metabolic acidosis with an increased anion gap, it is essential to consider inborn errors of metabolism and other disorders that can lead to the accumulation of acidic metabolites.

**Correct Answer: A.**