A 25-year-old woman of Bangladeshi origin presented with an insidious onset of swelling over her right knee. This had been present for three months following a fall. She had been treated previously with oral flucloxacillin by her general practitioner with no resolution of symptoms. Walking had become increasingly difficult. She had no other respiratory or systemic symptoms of note. She had no history of contact with infectious diseases and there had been no recent travel or family illness.
On examination, the patient had a large fluctuant prepatellar swelling with extensive surrounding erythema. There was a central punctum with a small amount of pus discharge. Range of movement was 0-110  but there was marked quadriceps wasting. The white cell count was normal, C-reactive protein (CRP) was 11mg/l and the erythrocyte sedimentation rate (ESR) was 30mm/hr. An x-ray showed a large prepatellar swelling but no obvious bony pathology (Fig 1).  
The patient was diagnosed with an infected prepatellar bursitis, and taken to theatre for incision and drainage. At the time of surgery, 10ml of pus was drained and sent for microbiology, growing Staphylococcus aureus. No note was made of any patella abnormality at the time of surgery. After clinical improvement and a week of intravenous antibiotics she was discharged.
A month later, the patient was admitted with a persistently painful discharging knee sinus. Respiratory examination revealed symmetrical breath sounds without crackles. There was no lymphadenopathy detectable on clinical examination. Chest x-ray showed no obvious pulmonary lesion. Further knee radiography showed a central nidus of necrotic bone in the patella (Fig 2). Magnetic resonance imaging (MRI) confirmed osteomyelitis (Fig 3). She was taken to theatre where a necrotic area of bone was found in the patella with granulomatous material surrounding it. There was no communication with the knee joint itself. A sinus excision and sequestrectomy was performed. Microbiology of the sequestered bone grew acid-fast bacilli, later confirmed as Mycobacterium tuberculosis.   
The patient had an uneventful recovery from surgery, and was commenced on oral antituberculous medication of pyrazinamide and ethambutol by the respiratory team. No primary lung focus was found and no tuberculosis (TB) positive contacts were identified. She made a good clinical recovery with no recurrence of infection. At six months postoperatively, she has no pain in the knee, a full range of movement and return of quadriceps bulk.