After consuming salmon lox bagel, the dog developed loose, watery diarrhea. Here's a categorized differential diagnosis:

Single Most Likely Diagnosis
- Gastroenteritis due to dietary indiscretion: This is the most likely diagnosis because the dog ate something unusual (salmon lox bagel), which can cause stomach upset and diarrhea in dogs.
Other Likely Diagnoses
- Food poisoning (e.g., Salmonella, E. coli): These bacteria can be present in raw or undercooked fish and can cause gastrointestinal symptoms in dogs.
- Pancreatitis: The high-fat content in salmon lox can trigger pancreatitis, an inflammation of the pancreas, which can lead to diarrhea and other gastrointestinal symptoms.
Do Not Miss Diagnoses
- Allergic reaction or anaphylaxis: Although rare, an allergic reaction to a component in the salmon lox bagel (e.g., fish, dairy, or sesame seeds) can cause severe symptoms, including diarrhea, and can be life-threatening if not promptly addressed.
- Intestinal obstruction or foreign body: If the dog ingested a large amount of bagel or other material, it could potentially cause an intestinal obstruction, which is a medical emergency.
Rare Diagnoses
- Infectious diseases (e.g., parvovirus, distemper): While possible, these infectious diseases are less likely given the recent ingestion of salmon lox bagel and the absence of other symptoms.
- Inflammatory bowel disease: This chronic condition can cause diarrhea, but it is less likely to be triggered by a single incident of dietary indiscretion.