Differential Diagnosis for Brown Tongue, Watery Diarrhea, and Abdominal Discomfort

Single Most Likely Diagnosis

• Oral candidiasis (thrush) with gastrointestinal infection: The presence of a brown, scrapable tongue is highly suggestive of oral candidiasis. When combined with watery diarrhea and abdominal discomfort, it points towards a gastrointestinal infection, possibly caused by a bacterial or viral pathogen, with the oral candidiasis being a separate but concurrent infection.

Other Likely Diagnoses

• Gastroenteritis: This condition, often caused by viral or bacterial pathogens, can lead to watery diarrhea and abdominal discomfort. The brown tongue could be related to poor oral hygiene during illness or a side effect of certain medications.
• Food poisoning: Similar to gastroenteritis, food poisoning can cause the gastrointestinal symptoms described. The oral findings might be secondary to the primary illness.
• Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic diarrhea and abdominal pain. Oral manifestations, including changes in the tongue, can occur in IBD due to malabsorption or as an extraintestinal manifestation.

Do Not Miss Diagnoses

• Clostridioides difficile (C. diff) infection: This bacterial infection can cause severe diarrhea and abdominal discomfort. It's crucial to consider C. diff, especially if the patient has recently been on antibiotics, as it can lead to life-threatening complications if not treated promptly.
• Toxic ingestion: Certain toxic ingestions can cause gastrointestinal symptoms along with oral changes. Identifying and treating the specific toxin is critical to prevent serious outcomes.

Rare Diagnoses

• Whipple's disease: A rare, systemic bacterial infection that can cause diarrhea, abdominal pain, and weight loss. Oral manifestations are less common but can include changes in the tongue.
• Pellagra: A condition caused by niacin deficiency, which can lead to dermatitis, diarrhea, and dementia. Oral changes, including a beefy red tongue that might appear brown due to secondary infections, can be part of the presentation.
• HIV/AIDS: In advanced stages, HIV can cause a variety of gastrointestinal symptoms, including diarrhea, and oral manifestations like oral candidiasis. The brown tongue could be related to opportunistic infections or poor immune status.