A 51 year old man presents with fever, abdominal pain and diarrhoea. He also gives a six month history of joint pains. An abdominal CT scan is normal. A small bowel biopsy shows periodic acid-Schiff positive rods within the macrophages. What is the most likely diagnosis?
1) Crohn's disease
2) Ulcerative colitis
3) Inflammatory bowel disease
4) Whipple's disease