RESUMEN
This is a case of a woman in her 50s with HIV and uncontrolled diabetes who presented to the emergency department with urinary retention and a painful gluteal cleft lesion, admitted for cellulitis. Since initial CT and soft tissue ultrasound (US) were negative for fluid collection, the care team was surprised to find her symptoms continued to progress despite intravenous antibiotics. Finally, MRI 9 days into her admission demonstrated a 12-cm perirectal horseshoe abscess. The patient was ultimately treated with incision and drainage with Penrose drain placement. This case demonstrates the importance of maintaining a high suspicion for horseshoe abscess, a complex form of ischiorectal fossa abscess which can be missed on CT and US imaging, and which may expand rapidly in immunosuppressed patients.