**Core Concept**
Anion gap (AG) is a calculated parameter used to assess the presence of unmeasured anions in the blood. It is calculated as AG = Na+ - (Cl- + HCO3-), where Na+ is the sodium concentration, Cl- is the chloride concentration, and HCO3- is the bicarbonate concentration. An increase in anion gap indicates the presence of an unmeasured anion, which can be due to various causes such as lactic acidosis, diabetic ketoacidosis, or ingestion of certain toxins.

**Why the Correct Answer is Right**
Ureterostomy is a surgical procedure that involves the diversion of urine flow from the bladder to the skin, bypassing the urethra. In ureterostomy, the urine is diverted to the skin, and the kidneys continue to produce urine. However, the urine produced in ureterostomy does not contain chloride ions, which are normally reabsorbed in the kidneys. As a result, the chloride concentration in the blood decreases, leading to a decrease in the anion gap. This is because the anion gap is calculated as Na+ - (Cl- + HCO3-), and a decrease in Cl- will result in a decrease in the anion gap.

**Why Each Wrong Option is Incorrect**
**Option B:** Ethylene glycol is a toxic compound that is commonly found in antifreeze. It is metabolized to glycolate, which is an unmeasured anion that increases the anion gap. Therefore, ethylene glycol ingestion will increase the anion gap.
**Option C:** Methylene glycol is a metabolite of ethylene glycol and also contributes to an increase in the anion gap.
**Option D:** Diabetic ketoacidosis is a complication of diabetes mellitus that results in the production of ketone bodies, which are acidic and increase the anion gap.

**Clinical Pearl / High-Yield Fact**
The anion gap is a useful parameter in the diagnosis of metabolic acidosis. However, it is essential to remember that certain conditions, such as ureterostomy, can decrease the anion gap due to the absence of chloride ions in the urine.

**✓ Correct Answer: A. Ureterostomy**