What condition is suggested in a six-week-old infant with projective vomiting based on a sagittal sonogram?

In the case of a six-week-old infant experiencing projectile vomiting, the condition most indicated by a sagittal sonogram is pyloric stenosis. Pyloric stenosis is a narrowing of the pylorus, the opening from the stomach into the small intestine. This condition leads to difficulty in the passage of food and contributes to symptoms like projectile vomiting, which is often seen in infants.

The sagittal sonogram typically reveals a thickened pyloric muscle, which can be measured for confirmation of the diagnosis. The classic "string sign" or "target sign" appearance is often observed in ultrasound images, which supports the diagnosis of pyloric stenosis.

Understanding this, one can see that cholecystitis, cholelithiasis, and appendicitis are conditions more often associated with older children or adults and present with different symptoms that would not typically lead to projectile vomiting in a six-week-old infant. Therefore, pyloric stenosis is the most relevant diagnosis in this clinical scenario, as it directly correlates with the age of the infant and the symptom of projectile vomiting demonstrated on imaging.