Reactive Arthritis Unmasked by Tuberculosis: A Rare Case of Poncet's Disease With Disseminated <scp>TB</scp> in a Malnourished Young Adult

Poncet's disease is a rare, non-destructive reactive arthritis that occurs in the presence of active tuberculosis (TB) without direct joint infection. It is often underrecognized in TB-endemic regions due to its clinical overlap with septic or autoimmune arthritis. We report the case of a 22-year-old undernourished female from rural Pakistan who presented with a 4-week history of bilateral knee pain and swelling, low-grade fever, dry cough, appetite loss, and weight loss. Physical examination revealed joint swelling without erythema, bilateral lung crepitations, and a BMI of 15.7 kg/m². Initial laboratory investigations showed anemia, elevated ESR and CRP, while autoimmune and infectious screens were negative. Synovial fluid analysis from the knee was sterile and non-purulent. Chest X-ray and GeneXpert confirmed active pulmonary TB. Abdominal imaging showed hepatic parenchymal changes and moderate ascites. Based on clinical, radiological, and serological findings, a diagnosis of Poncet's disease was made in accordance with Sharma's diagnostic criteria. The patient was initiated on standard anti-tubercular therapy (HRZE) and supportive nutritional management. Non-steroidal anti-inflammatory drugs (NSAIDs) were used initially. Marked clinical improvement was observed within 3 weeks, including resolution of joint pain and systemic symptoms. At follow-up, she showed weight gain and normalized joint function without relapse. This case highlights the importance of recognizing Poncet's disease as a differential diagnosis in patients with unexplained arthritis in TB-prevalent areas. Early identification can prevent misdiagnosis and avoid unnecessary use of immunosuppressants or prolonged antibiotics. Poncet's disease should be considered in patients with inflammatory arthritis and active TB, especially when synovial cultures are negative. Awareness and adherence to diagnostic criteria can aid in timely diagnosis and management.