A 45 year old lady, known to have rheumatoid arthritis, presented with tingling and numbness on the lateral three and half fingers. On examination there is wasting of the the thenar muscles and Tinel's sign is positive. What is the diagnosis?

The diagnosis is Carpal Tunnel Syndrome, which is likely exacerbated by her rheumatoid arthritis. The symptoms of tingling and numbness in the lateral three and a half fingers, along with thenar muscle wasting and a positive Tinel's sign (a tingling sensation with percussion over the median nerve), are classic signs of median nerve compression at the wrist. Rheumatoid arthritis is a known risk factor for carpal tunnel syndrome, as it can cause swelling and increased pressure in the carpal tunnel.