Giardiasis: This is a common cause of yellow, greasy, and foul-smelling stools due to the infection of the small intestine by the Giardia lamblia parasite. The symptoms often include diarrhea, abdominal cramps, and weight loss.

Malabsorption: Conditions such as celiac disease, pancreatic insufficiency, or liver disease can lead to malabsorption of fats, resulting in yellow, fatty stools.
Gastroesophageal Reflux Disease (GERD): Although more commonly associated with heartburn, severe GERD can lead to bile reflux, causing yellow or greenish stools.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause changes in stool color and consistency, including yellow or tarry stools, due to inflammation and bleeding in the gastrointestinal tract.

Upper Gastrointestinal Bleeding: A life-threatening condition that can cause tarry, black stools (melena) due to bleeding from the upper GI tract. Although the stool is described as tarry/yellow, it's crucial to consider the possibility of upper GI bleeding, especially if the patient is hemodynamically unstable.
Intestinal Obstruction: A condition where the intestine is partially or completely blocked, which can lead to severe abdominal pain, vomiting, and changes in stool color or consistency. Early diagnosis is critical to prevent complications.

Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, leading to changes in stool color and consistency, including yellow or fatty stools.
Carotenemia: A harmless condition caused by excessive consumption of carotenoid-rich foods (like carrots), leading to a yellow discoloration of the skin and potentially the stools.
Pseudomembranous Colitis: A rare condition caused by an overgrowth of Clostridioides difficile bacteria, leading to diarrhea, abdominal pain, and potentially changes in stool color or consistency.