Infectious mononucleosis mimicking Fitz-Hugh-Curtis Syndrome: The diagnostic complexity of concurrent Chlamydia trachomatis infection and false-positive syphilis serology.

Infectious mononucleosis (IM), typically caused by the Epstein-Barr virus, presents with a wide range of clinical manifestations. This diversity includes rare organ complications and the potential for false-positive syphilis serology results, making the diagnosis challenging. We report a rare and instructive case of IM presenting as acute acalculous cholecystitis (AAC). This case was complicated by concurrent false-positive syphilis serology results, prompting strong clinical suspicion for Fitz-Hugh-Curtis syndrome (FHCS). A 21-year-old woman presented with fever, right upper quadrant pain, and gallbladder wall thickening. The initial evaluation, including lymphocytosis with atypical lymphocytes, suggested IM. However, the clinical picture was obscured by a potent confounding factor: a false-positive syphilis serology and a concurrent diagnosis of pelvic inflammatory disease (PID) based on the presence of cervical motion tenderness and a positive Chlamydia trachomatis PCR result. These sexually transmitted infections-related indicators, when combined with the patient's right upper quadrant pain and liver dysfunction, created a diagnostic dilemma by strongly suggesting Fitz-Hugh-Curtis syndrome (FHCS), prompting empirical treatment. A false-positive syphilis serology was subsequently confirmed using other laboratory methods, establishing the final diagnosis of EBV-associated IM complicated by AAC, with a co-existing PID. This case highlights how IM can mimic other serious conditions such as FHCS, particularly when the rare complication of AAC is compounded by a strong diagnostic bias caused by false-positive syphilis serology results. Clinicians should be vigilant regarding the diverse clinical and laboratory manifestations of IM.