MRCP2-2849

A 28-year-old man presents with a 2-day history of facial weakness. He also reports a 1-month history of joint pain and stiffness, particularly affecting his elbows and knees. Three months previously, he had a flu-like illness with a red rash, fever and malaise. In the preceding 6 months he had travelled to Kenya and Massachusetts, USA for cross-country running competitions.

What is the most probable diagnosis?