Isolated Group A Streptococcal Psoas Abscess in an Immunocompetent Young Woman: A Rare Complication Following a Gynecological Procedure.

Psoas abscess is a rare but serious infection most commonly caused by Staphylococcus aureus or enteric organisms. Streptococcus pyogenes is an exceptionally rare etiology, reported only in isolated cases. Invasive group A streptococcus (GAS) infections may follow gynecological procedures and can occur in otherwise healthy individuals. This report highlights an atypical presentation of GAS bacteremia with psoas myositis and developing abscess in an immunocompetent patient. A 21-year-old previously healthy woman presented with a 10-day history of worsening bilateral lower-extremity pain, fever, and inability to ambulate, beginning two weeks after a medication-induced abortion. On presentation, she was febrile and tachycardic, with significant leukocytosis (32,000/μL) and elevated inflammatory markers. Given her severe pain and limited mobility, initial concern for neurologic pathology prompted a lumbar puncture, which demonstrated cerebrospinal fluid pleocytosis. Empiric therapy for meningitis was initiated. Blood cultures later grew Streptococcus pyogenes, confirming bacteremia, and antibiotics were narrowed accordingly. Neurologic evaluation ultimately deemed meningitis unlikely, with findings consistent with a parameningeal inflammatory response. CT of the abdomen and pelvis demonstrated inflammation of the psoas and iliacus muscles, with interval imaging showing a small developing psoas abscess that was not amenable to drainage. No alternative infectious source was identified. The patient was managed with targeted intravenous antibiotics and supportive care. She demonstrated progressive clinical improvement, regained independent ambulation, and was discharged on oral antibiotics with outpatient follow-up. GAS-associated psoas abscess is exceedingly rare, particularly in immunocompetent patients without traditional risk factors. In this case, a recent medication-induced abortion likely served as the portal of entry through mucosal disruption and hematogenous spread. Diagnosis was complicated by an atypical presentation mimicking meningitis, including sterile cerebrospinal fluid pleocytosis. Recognizing that a parameningeal inflammatory response can be a secondary manifestation of a nearby infection is essential; this understanding encourages clinicians to look beyond the central nervous system and avoid delays in identifying retroperitoneal sources. Management depends on the abscess size and clinical stability. This report adds to evidence suggesting that small abscesses may be successfully managed with antibiotics alone. Additionally, the absence of localizing gynecologic symptoms may obscure the source of infection, necessitating a high index of suspicion. This report demonstrates that Streptococcus pyogenes can cause deep musculoskeletal infection in healthy individuals and may present with misleading neurologic manifestations. Early recognition, appropriate imaging, and targeted therapy are critical. Clinicians should consider a deep soft-tissue infection in post-gynecologic patients presenting with severe pain and systemic symptoms, even in the absence of classic risk factors.