This is pyloric stenosis. Note the prominent hypertrophied muscle with elongation and narrowing of the pylorus at the gastric outlet on the left. Pyloric stenosis is uncommon, but is a cause for "projectile" vomiting in an infant about 3 to 6 weeks of age. Males are affected more than females. The overall incidence is approximately 3 per 1000 livebirths.

The "pacemaker" interstitial cells of Cajal (ICC) regulate motility, and contain the enzyme heme oxygenase-2 which generates carbon monoxide (CO) as a neurotransmitter to cause relaxation in adjacent smooth muscle cells. The lack of ICCs in pyloric stenosis results in deficient CO production leading to motility dysfunction.