This patient has pinprick sensory loss over the thumb in the C6 dermatome. A nerve root supplies sensation to a segment of skin known as a dermatome. Knowledge of the distribution of the dermatomes aids in the performance and interpretation of the sensory exam. C6 dermatomal pinprick loss is due to dysfunction of the sixth cervical nerve root and is also known as C6 radiculopathy. Nerve-root compression by a herniated disk or osteophyte is the most common cause of radiculopathy.