This patient has pinprick sensory loss over the lateral shin and big toe, in the L5 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. L5 dermatomal pinprick loss is due to dysfunction of the fifth lumbar nerve root and is also known as L5 radiculopathy. Nerve-root compression by a herniated disk or osteophyte is the most common cause of radiculopathy.