What diagnostic finding may indicate hypertrophic pyloric stenosis?

The correct choice is indicative of hypertrophic pyloric stenosis due to the characteristic ultrasound finding. In hypertrophic pyloric stenosis, the pylorus, which is the opening from the stomach into the small intestine, becomes severely thickened due to muscle hypertrophy. This thickening can be visualized on ultrasound as a "target" appearance or a thickened muscular layer of the pylorus, often exceeding 4 mm in thickness.

This imaging finding is critical for diagnosis since it directly correlates with the condition's presentation, which commonly includes projectile vomiting and dehydration in infants. The ultrasound serves as a non-invasive, efficient method to confirm the diagnosis before proceeding to surgical intervention.

The other options do not provide the same relevance to the diagnosis of hypertrophic pyloric stenosis. For instance, rapid gallbladder filling on a HIDA scan would suggest a different pathology related to gallbladder function rather than gastric outlet obstruction. A longitudinal incision along the history of the pylorus is more relevant to the surgical approach for correction, not a diagnostic tool. Endoscopic findings showing an obstructed duodenum would indicate another type of obstruction not specific to pyloric stenosis. Thus, the ultrasound finding of a thickened pylorus is the most