Final answer:
A 2-week old infant with a hard knot on the abdomen and projectile vomiting likely has pyloric stenosis, a condition involving the obstruction of the stomach's exit to the intestine.
Step-by-step explanation:
The most likely diagnosis for a 2 week old infant with a hard knot on the abdomen and projectile vomiting is pyloric stenosis. This condition involves the thickening of the pyloric sphincter, which can cause obstruction and forceful vomiting.
Pyloric stenosis typically presents in infants around 2-8 weeks old. Key symptoms include projectile vomiting, visible peristalsis, and a palpable "olive"-like mass in the abdomen. These symptoms arise because the thickened pylorus obstructs the passage of food from the stomach into the intestine, resulting in immediate and forceful vomiting after feeding.
Mom overfeeding the infant is less likely to cause a palpable mass and projectile vomiting. While a tumor can present with a mass, it would not typically cause projectile vomiting at such a young age. Celiac disease is very rare in infants as it generally presents after the introduction of gluten into the diet, which is not usually before 6 months of age.