Primary Septic Arthritis of The Knee due to Mycobacterium Tuberculosis in a Previously Healthy Child.

Pediatric septic arthritis can be a devastating disease. Often, the diagnosis can be challenging as autoimmune and infectious causes may present in a similar fashion. Thus, we present the case of a five-year-old male patient, from the Pacific coast of Colombia, with chronic morning knee pain and stiffness thought to be caused by an autoimmune disease. He presented with a mild effusion of the left knee, a flexed posture, and limited extension to 25°. Inflammatory markers demonstrated an infectious pattern. Autoimmune markers were negative. A diagnostic arthrotomy and lavage was conducted followed by microbial cultures, cell count, and gram staining. Polymerase chain reaction (PCR) of the joint fluid demonstrated mycobacterium tuberculosis. The patient was treated according to the national protocols and continued on to complete resolution.