What is the most likely diagnosis for a 28-year-old man who experienced weight loss, abdominal distension, and foul-smelling diarrhoea after a trip to India?

The symptoms presented—a combination of weight loss, abdominal distension, and foul-smelling diarrhea—strongly suggest an infection with Giardia lamblia, a common intestinal parasite that can cause giardiasis. This condition is particularly associated with regions where sanitation may be compromised, making travel to areas like India a potential risk factor for infection.

Foul-smelling diarrhea is a hallmark of giardiasis, often described as greasy in appearance due to malabsorption caused by the parasite. Weight loss and abdominal distension can also occur due to nutrient malabsorption and resultant gastrointestinal symptoms. The presentation fits well with the typical clinical picture of giardiasis, especially after recent travel to an endemic region.

While acute HIV seroconversion illness can present with gastrointestinal symptoms, it typically also includes systemic symptoms like fever and sore throat, and does not commonly lead to profound weight loss accompanied by specific gastrointestinal symptoms alone. Coeliac disease usually results in similar gastrointestinal symptoms but is less likely to present acutely following travel and would typically require a longer duration to manifest these symptoms significantly. Viral gastroenteritis would more commonly lead to self-limiting diarrhea but is not typically associated with prolonged weight loss or foul-smelling stools. Thus, given the travel history and symptomatology