Serum anion gap is increased in patients with which condition?

• A. Renal Tubular Acidosis
• B. Diabetes Alkalosis
• C. Metabolic Acidosis Due to Diarrhea
• D. Lactic Acidosis

Answer: (D)

An increased serum anion gap means there are more unmeasured anions (like organic acids) in the blood. The gap is calculated as Na minus (Cl plus HCO3). When lactate accumulates in conditions such as lactic acidosis, it adds to those unmeasured anions and pushes the gap higher. At the same time, bicarbonate is consumed to buffer the excess H+, further lowering HCO3 and widening the gap.

In contrast, metabolic acidosis from diarrhea or many forms of renal tubular acidosis is typically a normal anion gap (hyperchloremic) acidosis because the problem is bicarbonate loss or defective bicarbonate handling with chloride balancing, not accumulation of unmeasured acids. The term "diabetes alkalosis" isn’t a standard diagnosis, and if diabetic ketoacidosis were present, that would indeed raise the anion gap due to ketones, but the stated term does not reflect a high-gap scenario.

So, of the conditions listed, lactic acidosis best explains an increased anion gap.