What is the most likely diagnosis for a 21-year-old patient showing signs of abdominal guarding and rebound tenderness?

The presence of abdominal guarding and rebound tenderness suggests irritation of the peritoneum, which typically occurs in cases of inflammation or acute abdominal conditions. In this scenario, appendicitis is the most likely diagnosis.

Appendicitis occurs when the appendix becomes inflamed, often due to an obstruction or infection. As the condition progresses, the inflammation can irritate the surrounding peritoneum, leading to guarding—where the abdominal muscles tighten to protect the inflamed area—and rebound tenderness, which is a classic sign where pain is felt upon releasing pressure on the abdomen. These signs indicate a more serious condition that may require urgent surgical intervention.

In contrast, while diverticulitis might also cause abdominal pain and could lead to guarding, it's more commonly associated with the left lower quadrant of the abdomen and typically affects older adults. Gastritis and peptic ulcer disease can cause abdominal discomfort but are less likely to produce the specific signs of rebound tenderness and guarding since these conditions do not usually lead to peritoneal irritation to the same extent as appendicitis. Hence, the combination of abdominal guarding and rebound tenderness points more definitively to appendicitis as the diagnosis in this case.