Gastroenteritis: This is often the most common cause of green stool, resulting from a viral or bacterial infection that leads to inflammation of the stomach and intestines. The green color can be due to the rapid transit of stool through the intestines, which doesn't allow for the complete breakdown of bile, resulting in its green pigment being visible in the stool.

Food Intake: Consuming foods or drinks with green food coloring or certain vegetables like spinach and green beans can temporarily turn stool green.
Malabsorption: Conditions that lead to malabsorption, such as celiac disease or pancreatic insufficiency, can result in green stool due to the inadequate digestion and absorption of nutrients and bile salts.
Antibiotic Use: The use of broad-spectrum antibiotics can alter the gut flora, leading to changes in stool color, including green.

Biliary Obstruction: Although less common, biliary obstruction (e.g., due to gallstones or a tumor) can cause green stool if the obstruction leads to bile leaking into the stool. Missing this diagnosis could lead to severe complications, including jaundice and infection.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause green stool due to inflammation and ulceration of the bowel wall, leading to rapid transit and incomplete digestion of bile.
Gastrointestinal Infection: Certain infections, such as those caused by Salmonella or E. coli, can lead to green stool and are critical to diagnose and treat promptly to prevent complications.

Giardiasis: An intestinal infection caused by the parasite Giardia lamblia, which can lead to malabsorption and changes in stool color, including green.
Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, leading to changes in stool color among other symptoms.
Cystic Fibrosis: A genetic disorder that can lead to malabsorption and pancreatic insufficiency, potentially resulting in green stool due to the abnormal digestion and absorption of fats and bile salts.