MRCP2-3036

A 32 year-old male patient complains of gradual onset weakness in his right hand for the past week. Being right hand dominant, he is finding it difficult to perform simple tasks like unlocking a door with a key.

Upon examination, he displays significant weakness in flexing his thumb at the interphalangeal joint and flexion of the index and middle finger. No sensory deficit is detected.

What is the probable diagnosis?