What is the most likely diagnosis for a 21-year-old patient presenting with rebound tenderness at McBurney's point?

Rebound tenderness at McBurney's point is a classic clinical sign associated with appendicitis, particularly indicative of inflammation in the appendix. The presentation of pain that worsens upon the release of pressure suggests irritation of the peritoneum, which is often due to inflammatory conditions originating in the abdomen. In appendicitis, the appendix becomes inflamed, and as it progresses, it can irritate the surrounding peritoneal lining, leading to rebound tenderness.

In contrast, other conditions listed, such as gastritis, typically present with symptoms related to the stomach and do not focus on localized tenderness in the right lower quadrant. Diverticulitis primarily affects the colon, usually presenting with left lower quadrant pain, and does not typically elicit rebound tenderness at McBurney's point. Pelvic inflammatory disease is more associated with lower abdominal pain and tenderness but does not specifically correlate with rebound tenderness in that location. Therefore, the signs and symptoms observed in the scenario point clearly towards appendicitis as the most likely diagnosis.