Celiac Disease: This is the most likely diagnosis given the focus on malabsorption. Celiac disease is a chronic immune-mediated disorder caused by a reaction to gluten, leading to small intestine mucosal damage and malabsorption of nutrients. Typical presentation includes diarrhea, abdominal pain, weight loss, and fatigue.

Irritable Bowel Syndrome (IBS): Although IBS does not typically cause malabsorption, it can present with similar gastrointestinal symptoms such as diarrhea, abdominal pain, and bloating, making it a consideration in the differential diagnosis.
Lactose Intolerance: This condition, characterized by the inability to digest lactose due to lactase deficiency, can lead to symptoms like diarrhea, bloating, and gas after consuming lactose-containing products.
Small Intestine Bacterial Overgrowth (SIBO): SIBO can cause malabsorption and symptoms similar to celiac disease, including diarrhea, abdominal pain, and weight loss, due to an excessive growth of bacteria in the small intestine.
Chronic Pancreatitis: This condition can lead to malabsorption due to pancreatic enzyme insufficiency, presenting with symptoms like steatorrhea (fatty stools), weight loss, and abdominal pain.

Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption, weight loss, diarrhea, and abdominal pain. It's crucial to diagnose Whipple's disease early, as untreated cases can be fatal.
Lymphoma: Both primary intestinal lymphoma and secondary involvement of the intestine by systemic lymphoma can cause malabsorption and gastrointestinal symptoms. Early diagnosis is critical for effective treatment.
Crohn's Disease: A type of inflammatory bowel disease that can affect any part of the gastrointestinal tract, leading to malabsorption, diarrhea, abdominal pain, weight loss, and potentially life-threatening complications if not properly managed.

Tropical Sprue: A rare condition characterized by malabsorption and gastrointestinal symptoms similar to celiac disease, typically acquired in tropical regions.
Abetalipoproteinemia: A very rare genetic disorder affecting the absorption of fat and fat-soluble vitamins, leading to severe malabsorption and specific clinical findings like acanthocytosis.
Short Bowel Syndrome: Although not a disease per se, this condition results from a significant loss of intestinal tissue, which can be due to various causes, including surgical removal. It leads to severe malabsorption and requires specialized management.