Not All Pseudomembranous Colitis is Caused by Clostridium difficile

A 43-year-old woman presented to hospital with a two-day history of acute left lower quadrant pain, nonbloody diarrhea and one episode of bilious emesis. She had a history of complicated systemic lupus erythematosus including end-stage renal disease requiring chronic hemodialysis. She had a remote history of pulmonary tuberculosis that had been appropriately treated. Four months earlier, she was admitted with community-acquired pneumonia and treated with 10 days of levofloxacin. There was no recent travel history, infectious contacts or suspicious food consumed, and no family history of inflammatory bowel disease. She immigrated to Canada from Cambodia in 1988, but had never returned to visit. Her medications included long-standing prednisone (15 mg daily), azathioprine, levothyroxine, carvedilol, pantoprazole, acetylsalicylic acid and pravastatin. Her examination revealed a fever of 39°C, pulse rate of 103 beats/min, a left sternal heave, a grade 2/6 systolic murmur at the left lower sternal border radiating to the apex with an S4, and a tender abdomen in the left lower quadrant without rebound or guarding. Laboratory results revealed a total leukocyte count of 12.4×109/L with a normal differential, and normal liver enzymes and serum lipase. Her blood cultures were negative. Abdominal x-rays showed mural thickening in the splenic flexure and descending colon, but no evidence of obstruction. An abdominal computed tomography scan revealed evidence of pancolitis. She underwent sigmoidoscopy which demonstrated only pseudomembranes on rectal biopsy. However, stool enzyme immunoassays for Clostridium difficile toxins A and B (C difficile ToxA-BII, Techlab, USA) were repeatedly negative; she did not respond clinically to oral metronidazole, and a repeat sigmoidoscopy one week later was unchanged. A diagnostic test was performed. What is the diagnosis?