Appendicitis: This is often the first consideration in acute abdomen, especially in younger patients, due to its high prevalence and the potential for severe complications if not promptly treated.

Gastroenteritis: Inflammation of the gastrointestinal tract, usually caused by infection, which can present with acute abdominal pain, diarrhea, and vomiting.
Intestinal Obstruction: A condition where there is a blockage that prevents the normal flow of the products of digestion, including food, fluid, and gas, which can cause severe abdominal pain, vomiting, and constipation.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones, which can cause right upper quadrant abdominal pain, fever, and leukocytosis.
Pancreatitis: Inflammation of the pancreas, which can be acute or chronic, and is characterized by severe abdominal pain, vomiting, and elevated pancreatic enzymes.

Ectopic Pregnancy: A pregnancy in which a fertilized egg implants outside the uterus, which can cause severe abdominal pain and vaginal bleeding, and is a medical emergency.
Ruptured Aortic Aneurysm: A condition where the aorta ruptures, leading to severe abdominal and back pain, and is immediately life-threatening.
Perforated Visculature (e.g., perforated ulcer): A hole in the wall of the stomach or intestine, which can lead to peritonitis, an infection of the abdominal cavity, and is a surgical emergency.
Ovarian Torsion: A condition where the ovary twists around its ligaments, cutting off blood supply, which can cause severe abdominal pain and is a surgical emergency.

Mesenteric Ischemia: A condition where there is insufficient blood flow to the intestine, which can cause severe abdominal pain, and is often associated with atherosclerosis, embolism, or venous thrombosis.
Hernias (e.g., inguinal, femoral, or internal): Protrusions of an organ or tissue through a weakened area in the muscle or connective tissue that normally holds it in place, which can cause abdominal pain and discomfort.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause acute abdominal pain during a flare, although they are more chronic in nature.
Abdominal Malignancies: Various cancers that can cause acute abdominal symptoms, though they are less common causes of acute abdomen compared to other conditions listed.