Small Bowel Obstruction: This is the most common cause of air-fluid levels in the abdomen on an X-ray. It occurs when there is a mechanical blockage in the small intestine, preventing normal flow of intestinal contents, which can be due to various reasons such as adhesions, hernias, or tumors.

Volvulus: A condition where a loop of intestine twists around itself and the mesentery that supports it, cutting off blood flow and potentially leading to tissue death. It can cause air-fluid levels due to the obstruction and possible ischemia.
Intussusception: A condition where a part of the intestine slides into an adjacent part, one segment into another, which can cause bowel obstruction and potentially lead to air-fluid levels on an X-ray.
Paralytic Ileus: A temporary cessation of the normal contractions of the bowel muscles, leading to a build-up of food, fluids, and gas, which can result in air-fluid levels.

Mesenteric Ischemia: A serious condition where there is an interruption of blood flow to the intestines, which can lead to tissue death. It may present with air-fluid levels among other symptoms and is critical to diagnose early.
Perforated Viscus: A hole in the wall of the intestine or stomach, allowing air to escape into the abdominal cavity, which can be life-threatening and requires immediate surgical intervention.

Internal Hernia: A rare condition where there is a protrusion of an organ or tissue through a defect within the peritoneal cavity, which can cause bowel obstruction and air-fluid levels.
Gallstone Ileus: A rare condition where a gallstone enters the bowel through a fistula and causes a mechanical obstruction, potentially leading to air-fluid levels on an X-ray.