Image of the Week: What disorder is a risk factor for this presentation?

A 71-year-old man with a history of atrial fibrillation and hypertension presents to the emergency department with a one-day history of severe abdominal pain. On examination, his vital signs are stable and laboratory findings reveal an expected INR of 2.8 but are otherwise unremarkable. A palpable tender mass can be felt in the right lower quadrant of his abdomen. When ...