A Rare Case of Acalculous Cholecystitis Secondary to Infectious Mononucleosis.

Infectious mononucleosis (IM) is a syndrome characterized by malaise, headache, lymphadenopathy, pharyngitis, rash, enlarged spleen, and fever. Epstein-Barr virus (EBV) is the most common cause of IM, though other organisms, such as cytomegalovirus (CMV), can cause it as well. Complications include transient hepatitis, splenic rupture, peritonsillar abscess, and other organ damage. This case study illustrates a case of acalculous cholecystitis caused by IM - a rare complication - with 44 EBV-associated acute acalculous cholecystitis (AAC) cases reported, and far fewer cases of CMV-associated AAC, though the exact number varies in the literature. The patient was a 23-year-old woman who presented to the Emergency Department (ED) due to acute-onset right upper quadrant (RUQ) abdominal pain with radiation to her right upper back, accompanied by nausea, sore throat, and fevers. Laboratory workup and abdominal ultrasound (US) suggested AAC secondary to IM, and the patient was treated supportively. Though AAC is an uncommon complication overall, similar cases have been reported in the literature that show overlapping clinical features; however, variability exists in laboratory findings, imaging, and associated symptoms. Presentations can differ depending on the patient's age, immune response, and the specific viral etiology, as will be described later in the discussion. This case highlights the importance of considering mononucleosis in the differential diagnosis of AAC, particularly in young adults presenting with symptoms consistent with a viral prodrome. Awareness of this rare complication can guide appropriate management, help avoid unnecessary interventions such as surgery, and prevent unneeded radiation and antibiotic exposure.