Case of the month. TB or not TB

A 38-year-old Asian man presented with a short history of fever, night sweats and chest pains. He felt weak in the legs and had particular dif®culty in rising from a sitting position and in walking. There was no relevant past or family history. On examination there was tender swelling of both quadriceps muscles but there were no neurological signs or muscle wasting and the chest was clear. C-reactive protein (CRP) was mildly elevated at 19 mg l and serum angiotensin-converting enzyme was normal. A chest radiograph and CT showed normal lung parenchyma but mediastinal and hilar lymphadenopathy, which led to mediastinoscopy for lymph node biopsy. The biopsy showed multiple granulomas suggestive of tuberculosis (TB) although no acid-fast bacilli (AFB) were seen or cultured. The patient was commenced on anti-tuberculous treatment but, after 6 months therapy, he continued to complain of leg weakness and night sweats and then developed lumpiness in both quadriceps. His creatinine kinase (CK) was elevated at 327 U l and CRP remained high. An MR scan of the thighs was performed (Figure 1). What is the most likely diagnosis?