Acute Appendicitis: This is often the first consideration for extreme abdominal pain, especially if the pain starts near the belly button and then moves to the lower right abdomen. The severity of pain and potential for rapid progression to a surgical emergency make it a prime candidate for the most likely diagnosis.

Kidney Stones (Nephrolithiasis): Severe, crampy pain that may radiate from the flank to the groin, often accompanied by nausea, vomiting, and sometimes blood in the urine.
Diverticulitis: Inflammation of the diverticula in the digestive tract, which can cause severe abdominal pain, usually on the lower left side, along with fever and changes in bowel habits.
Gastrointestinal Obstruction: Blockage of the intestine that can cause severe abdominal pain, vomiting, and constipation. The pain can be colicky, coming and going at regular intervals.
Pancreatitis: Inflammation of the pancreas, which can cause severe, persistent pain in the upper abdomen that may radiate to the back, along with nausea and vomiting.

Ruptured Aortic Aneurysm: A life-threatening condition where the aorta tears, leading to severe abdominal and back pain. It's crucial to consider this in older patients or those with a history of aortic aneurysm.
Ectopic Pregnancy: A pregnancy that develops outside the uterus, often causing severe abdominal pain, vaginal bleeding, and sometimes shoulder pain due to blood irritating the diaphragm. This is a critical diagnosis to consider in women of childbearing age.
Perforated Viscus: A hole in the wall of the stomach or intestine, leading to peritonitis (inflammation of the lining of the abdominal cavity), which can cause severe abdominal pain, tenderness, and rigidity.
Mesenteric Ischemia: Insufficient blood flow to the intestines, which can cause severe abdominal pain out of proportion to the physical examination findings, often with a history of cardiovascular disease.

Intussusception: A condition where a part of the intestine slides into an adjacent part, potentially causing bowel obstruction and severe abdominal pain, more common in children but can occur in adults.
Inflammatory Bowel Disease (IBD) Flare: Conditions like Crohn's disease or ulcerative colitis can cause severe abdominal pain during a flare, along with diarrhea, weight loss, and sometimes blood in the stool.
Abdominal Migraine: A condition that causes recurrent episodes of severe abdominal pain, often accompanied by other migraine symptoms like headache or nausea, more commonly seen in children.
Porphyria: A group of disorders that can cause severe abdominal pain, along with neurological symptoms and sometimes psychiatric disturbances, due to a deficiency in heme synthesis.