Question

The metabolic derangement in congenital pyloric stenosis is:

• A. Hypochloremic alkalosis
• B. Hyperchloremic alkalosis
• C. Hyperchloremic acidosis
• D. Hypochloremic acidosis

Hint:

Vomiting stomach acid removes H+ and Cl-, so pyloric stenosis causes hypochloremic alkalosis.

Answer 1

The Correct Option is A

Pyloric stenosis blocks the stomach outlet, so the baby keeps bringing up stomach juice by forceful vomiting. Stomach juice is mostly hydrochloric acid, so tracking what leaves the body in that vomit tells us the acid-base picture.

1. Hypochloremic alkalosis: vomiting removes acid (hydrogen ions) and chloride together. Losing acid raises blood pH, giving alkalosis, and losing chloride drops the blood chloride, giving the hypochloremic part. This matches the vomiting pattern exactly.
2. Hyperchloremic alkalosis: chloride cannot be high when it is being vomited out in large amounts, so this does not fit.
3. Hyperchloremic acidosis: this pattern is seen with diarrhea or kidney bicarbonate loss, not with loss of stomach acid, so it is the wrong direction on both counts.
4. Hypochloremic acidosis: the chloride part is right, but losing stomach acid should raise the pH, not lower it, so calling it acidosis is wrong.

Dehydration from the vomiting also makes the kidney hold sodium and water while dumping potassium and acid in the urine, which is why the classic lab picture is described fully as hypochloremic, hypokalemic metabolic alkalosis.

Let's summarize:

• Vomiting stomach acid removes both hydrogen and chloride ions.
• The result is a rise in pH (alkalosis) with a fall in blood chloride (hypochloremic).

So the metabolic derangement in congenital pyloric stenosis is hypochloremic alkalosis.