A Man with Left Knee Pain

Five years ago, he received intravesical bacille Calmette–Guérin (BCG) as treatment for bladder cancer. One year ago, he had a methicillin-sensitive Staphylococcus aureus (MSSA) foot ulcer for which he received antibiotics. Current exam was notable for a left knee effusion with tenderness of the medial joint line. Imaging (see Figure 1) showed cortical erosions, osteolysis, synovitis (yellow arrow), and a juxta-articular soft-tissue mass (white arrow) and fluid collection (black arrow). Arthrocentesis was performed, with bloody fluid aspirated and analysis showing neutrophil-predominant leukocytosis.