What is the diagnosis for a middle-aged man with acute monoarticular joint pain and bilateral Bell's palsy?

The presentation of acute monoarticular joint pain combined with bilateral Bell's palsy strongly suggests Lyme disease. This condition is caused by the bacteria Borrelia burgdorferi, which is transmitted through the bite of infected ticks. Lyme disease is known for its characteristic symptoms, including erythema migrans (a particular type of rash) and neurological manifestations such as cranial nerve palsies, including Bell's palsy.

In this scenario, the joint pain indicates an inflammatory response, which can occur as part of the arthritic component of Lyme disease. While other conditions can cause joint pain or cranial nerve palsies separately, the combination of acute joint pain and bilateral facial paralysis aligns more closely with Lyme disease than with other conditions. Conditions like rheumatoid arthritis and reactive arthritis typically do not present with neurological symptoms such as Bell's palsy. Meanwhile, gout would have a different clinical presentation, often involving the big toe and typically not presenting with neurological deficits.

Thus, when evaluating the symptoms together, particularly the unusual combination of monoarticular joint pain alongside bilateral Bell's palsy, Lyme disease emerges as the most fitting diagnosis.