A 71-year-old male presents to clinic with complaints of numbness and tingling in his right hand ongoing for 2 weeks duration. The patient does not recall any inciting events, nor has he observed any aggravating or alleviating factors. Further questioning reveals that the patient has had chronic neck pain. Vitals are within normal limits. Physical examination reveals decreased sensation of the first and second digits as well as 4/5 strength in flexion at the elbow of the right upper extremity. Which nerve innervates the muscle primarily responsible for elbow flexion?