Gastroenteritis: This is often the most common cause of abdominal tenderness, especially if the patient has a history of recent travel, food poisoning, or viral infection. The symptoms typically include diarrhea, vomiting, and abdominal cramps.

Irritable Bowel Syndrome (IBS): Characterized by chronic abdominal pain, bloating, and alteration of bowel habits in the absence of any mechanical, inflammatory, or biochemical abnormalities. It's a common condition that can cause significant discomfort.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic abdominal tenderness, diarrhea, weight loss, and sometimes blood in the stool.
Peptic Ulcer Disease: Ulcers in the stomach or duodenum can cause abdominal tenderness, especially in the upper abdomen, and may be associated with nausea, vomiting, and bleeding.
Constipation: Severe constipation can lead to abdominal tenderness, bloating, and discomfort.

Appendicitis: Although not the most common diagnosis, appendicitis is a medical emergency that requires prompt surgical intervention. It typically presents with severe abdominal tenderness that initially may be diffuse but often localizes to the right lower quadrant.
Ectopic Pregnancy: In women of childbearing age, an ectopic pregnancy can cause severe abdominal tenderness, especially if it has ruptured. This is a life-threatening condition that requires immediate medical attention.
Perforated Viscus: A hole in the wall of the stomach or intestine can lead to peritonitis, an infection of the abdominal cavity, which is a medical emergency.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal tenderness, nausea, vomiting, and may be associated with elevated pancreatic enzymes.

Intestinal Obstruction: A blockage in the intestine can cause severe abdominal tenderness, vomiting, and constipation. It's less common but can be a medical emergency.
Mesenteric Ischemia: A reduction in blood flow to the intestines can cause abdominal tenderness, nausea, and may lead to bowel infarction if not promptly treated.
Endometriosis: Although more commonly associated with pelvic pain, endometriosis can cause abdominal tenderness, especially during menstruation.
Abdominal Malignancies: Various cancers (e.g., stomach, colon, ovarian) can cause abdominal tenderness, although they are less common causes compared to the other conditions listed.