Crohn Disease
An 18-year-old woman has 10 months of loose non-bloody stools with crampy right lower quadrant pain, and she has lost 6 kg. She has had no rectal bleeding. There is tenderness in the right iliac fossa. Haemoglobin is 100 g/L, mean corpuscular volume 100 fL, platelets 540 × 10⁹/L, vitamin B12 74 pmol/L (reference 120 to 660), and folate is low. Which of the following is the most likely diagnosis?
- A.Celiac disease
- B.Crohn diseaseCorrect
- C.Ulcerative colitis
- D.Whipple disease
Explanation
The combination of chronic non-bloody diarrhoea, right lower quadrant pain, weight loss, thrombocytosis as an acute phase reactant, and vitamin B12 deficiency points to inflammatory disease of the terminal ileum, which is the classic site of Crohn disease. Cobalamin is absorbed exclusively in the terminal ileum bound to intrinsic factor, so ileal inflammation or resection is one of the few gastrointestinal causes of true B12 deficiency in a young patient; the resulting macrocytosis, compounded here by folate deficiency from generalised malabsorption, explains the mean corpuscular volume of 100 fL. Crohn disease is transmural and segmental, frequently spares the rectum, and therefore often presents without visible blood. Celiac disease causes proximal small bowel villous atrophy and typically produces iron and folate deficiency with preserved B12 absorption, and it is not usually associated with focal right iliac fossa tenderness. Ulcerative colitis is a mucosal disease that begins in the rectum and almost always causes bloody diarrhoea with tenesmus, and it does not impair ileal B12 uptake except in rare backwash ileitis. Whipple disease is a rare Tropheryma whipplei infection of middle-aged men presenting with arthralgia, fever, and neurological signs, which do not fit this patient. Confirmation requires ileocolonoscopy with biopsies showing focal chronic inflammation and non-caseating granulomas, supported by faecal calprotectin and cross-sectional enterography. Chronic disease of this severity also warrants screening for iron deficiency, which can mask macrocytosis and yield a normal mean corpuscular volume.
Why each option
- A.
- Celiac disease damages the proximal small bowel and causes iron and folate deficiency; vitamin B12 absorption in the terminal ileum is usually preserved.
- B.
- Correct. Terminal ileal Crohn disease explains non-bloody diarrhoea, right lower quadrant pain, weight loss, thrombocytosis, and vitamin B12 deficiency.
- C.
- Ulcerative colitis is a rectally based mucosal colitis that characteristically causes bloody diarrhoea and tenesmus and does not impair ileal B12 uptake.
- D.
- Whipple disease is a rare systemic infection with arthralgia, fever, and neurological features, typically in middle-aged men, and does not match this presentation.
Reference: Harrison's Principles of Internal Medicine, 21st ed., 2022; ECCO Guidelines on Crohn's Disease Diagnosis, 2023
This is one of 3,306 questions in the MedBoardSA SMLE bank, with timed test mode, tutor mode, and progress tracking by topic.
Practise the full bank