Mr. M.B., a 37-year-old engineer, presents to the emergency department (ED) with unrelenting diarrhea and severe joint pain. He was well until 4 months ago, when he developed non-bloody diarrhea associated with lower abdominal cramps. These cramps preceded defecation and were relieved afterwards. Since he had been diagnosed with irritable bowel syndrome a few years earlier -- a diagnosis that was based on symptoms only -- he assumed that this was a recurrence. In response, he reduced his stress level at work, took loperamide and increased his fibre intake -- to no avail.

There was no overt rectal bleeding or melena, but about 2 months after the onset of diarrhea, he noted bilateral ankle pain associated with swelling and redness. Within a week, the symptoms had crept to his knees, fingers and wrists. He also had lost over 7 kg by then.

His family physician prescribed lumiracoxib, which helped with the joint pain and swelling, but only made the diarrhea worse. His labs showed an elevated platelet count of 1,360 x 10⁹/L and a microcytic anemia, manifested as a hemoglobin of 61 g/L. His erythrocyte sedimentation rate was high at 111 mm/h and C-reactive protein was markedly elevated at 140 mg/L. Antinuclear antibodies and rheumatoid factor were negative. His doctor sent him to the ED for further investigations.

Mr. M.B. doesn't smoke and isn't taking any medications. He's in a monogamous heterosexual relationship and has no history of blood transfusions, illicit drug use or recent travel. On exam, there is generalized muscle wasting and bilateral ankle effusions. The patient is unable to walk without using crutches.

He has a serum albumin of 14 g/L. Liver profile is unremarkable and the same is true for his urinalysis, electrolytes, creatinine and blood urea nitrogen. Serum iron is low at 4 µmol/L, with a reduced iron saturation of 17% and an increased serum ferritin at 768 µg/L. Vitamin B₁₂ and folate levels are within normal limits; chest and abdominal x-rays are unremarkable. What's the problem?