Question

All of the following cause high anion gap metabolic acidosis except:

• A. Lactic acidosis.
• B. Salicylate poisoning.
• C. Ethylene glycol poisoning.
• D. Ureterosigmoidostomy.

Hint:

Ureterosigmoidostomy causes chloride gain and bicarbonate loss, so the gap stays normal.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We need to find the one option that does NOT cause a high anion gap metabolic acidosis, meaning it either causes acidosis with a normal anion gap, or does not fit this category at all.

Step 2: Key Concept:
The anion gap is calculated as
\[ \text{Anion gap} = Na^+ - (Cl^- + HCO_3^-) \]
A high anion gap acidosis occurs when an unmeasured acid builds up in the blood, common causes are remembered by the mnemonic MUDPILES: methanol, uremia, diabetic ketoacidosis, propylene glycol, isoniazid/iron, lactic acidosis, ethylene glycol, and salicylates.
A normal (or "hyperchloremic") anion gap acidosis happens when bicarbonate is lost directly, and chloride rises to take its place, keeping the gap normal.

Step 3: Detailed Explanation:
Lactic acidosis adds lactic acid to the blood, an unmeasured acid, so it raises the anion gap.
Salicylate poisoning produces salicylic acid and other organic acids, also unmeasured, so it raises the anion gap.
Ethylene glycol poisoning is broken down into glycolic and oxalic acid, both unmeasured acids, so it too gives a high anion gap.
Ureterosigmoidostomy connects the ureters to the sigmoid colon. The colon then reabsorbs chloride from urine in exchange for bicarbonate, which is then lost in stool. This lowers bicarbonate but raises chloride, keeping the anion gap normal, this is a classic cause of hyperchloremic (normal anion gap) metabolic acidosis, not high anion gap.

Step 4: Why the other options are wrong:
Options 1, 2 and 3 are textbook members of the MUDPILES list of high anion gap causes, so each of them is correctly linked to a raised anion gap, making them incorrect choices for this "except" question.

Step 5: Final Answer:
Ureterosigmoidostomy causes a normal anion gap (hyperchloremic) metabolic acidosis, not a high anion gap acidosis.
\[ \boxed{\text{Ureterosigmoidostomy}} \]