Final answer:
The most likely affected spinal nerve root in a patient with weak wrist extension, thumb and index finger paresthesias, and diminished triceps reflex is the median nerve.
Step-by-step explanation:
The symptoms described in the question suggest a compression of the median nerve in the wrist, which is associated with a condition called carpal tunnel syndrome.
Carpal tunnel syndrome occurs when the median nerve becomes compressed between the carpal bones in the wrist. The main symptoms include pain, numbness, and tingling in the thumb and first two fingers, as well as weakness in wrist extension. This compression can also lead to a diminished triceps reflex.