What is the primary diagnosis for an infant presenting with anorexia, periumbilical pain followed by RLQ pain, and symptoms of rebound tenderness?

The presentation of anorexia, periumbilical pain that later localizes to the right lower quadrant (RLQ), accompanied by rebound tenderness, strongly indicates appendicitis. In infants, the clinical symptoms can sometimes manifest differently than in older children or adults. The initial diffuse abdominal pain may present around the umbilical region due to the nature of visceral pain, which is often poorly localized. However, as inflammation progresses, the pain localizes to the RLQ, where the appendix is situated.

Rebound tenderness is a key sign of peritoneal irritation, which often occurs in cases of appendicitis due to the inflamed appendix causing irritation to the surrounding peritoneum. This combination of symptoms—anorexia, initial periumbilical pain, dereased appetite followed by localized RLQ pain, and the presence of rebound tenderness—aligns with the typical clinical presentation of appendicitis in an infant.

The other conditions, such as colic, gastroesophageal reflux, and pyloric stenosis, usually present with different symptom patterns and do not typically cause the specific progression of pain localization or the classic sign of rebound tenderness associated with appendicitis. Therefore, the clinical picture presented in the question is best characterized as indicative of