A 27-year-old male presents with abdominal pain and lies very still; any attempt to sit up increases pain. Which condition is most likely?

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Peritoneal irritation from inflammation makes the peritoneum extremely sensitive to movement. When the lining is inflamed, even slight shifts in position stretch the irritated tissue and sharply increase pain, so patients instinctively lie still and guard their abdomen to minimize motion. This pattern—severe, constant abdominal pain with patient immobility or guarding—is classic for peritonitis and helps distinguish it from other abdominal problems.

In contrast, gallstones typically cause episodic right upper-quadrant pain related to meals; pancreatitis presents with epigastric pain that often radiates to the back and may come with a more diffuse tenderness, but not the marked immobility driven by peritoneal irritation. Appendicitis can involve guarding, but its pain is usually periumbilical shifting to the right lower quadrant rather than a general, motion-sensitive tenderness.