## **Core Concept**
Normal-anion-gap metabolic acidosis, also known as hyperchloremic metabolic acidosis, is characterized by a decrease in bicarbonate levels without an increase in the anion gap. This condition often results from the loss of bicarbonate or the failure of the kidneys to reabsorb or regenerate bicarbonate.

## **Why the Correct Answer is Right**
The correct answer, **D. Renal tubular acidosis (RTA)**, is right because certain types of RTA, particularly Type 1 (distal) RTA and Type 2 (proximal) RTA, lead to a normal-anion-gap metabolic acidosis. In Type 1 RTA, the kidneys are unable to acidify the urine properly, leading to a loss of bicarbonate. In Type 2 RTA, there is a failure of the proximal tubule to reabsorb bicarbonate. Both conditions result in metabolic acidosis with a normal anion gap.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Diabetic ketoacidosis (DKA) is incorrect because it causes a high-anion-gap metabolic acidosis due to the accumulation of ketoacids.
- **Option B:** Lactic acidosis is incorrect because it also results in a high-anion-gap metabolic acidosis due to the accumulation of lactic acid.
- **Option C:** Chronic kidney disease (CKD) can cause a high-anion-gap metabolic acidosis as a result of the kidneys' reduced ability to excrete organic and inorganic acids.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in normal-anion-gap metabolic acidosis, the chloride level is often elevated because chloride replaces bicarbonate to maintain electroneutrality. This can be a helpful diagnostic clue. For example, in the context of RTA, the urine pH can help differentiate between types: in Type 1 RTA, the urine pH is inappropriately alkaline (>5.5) despite systemic acidosis.

## **Correct Answer: D. Renal tubular acidosis.**