I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 24-year-old woman is brought to the physician 1 month after she was involved in a motor vehicle collision that left her weak and unable to walk. Physical examination shows weakness of both hands and atrophy of the intrinsic hand muscles bilaterally. There is weakness and increased muscle tone of the lower extremities on passive range of motion. Deep tendon reflexes are normal at the biceps and triceps bilaterally and are increased at the knees and ankles. Babinski sign is present bilaterally. Sensation to pinprick is absent at and below the level of the clavicles.
The lesion in this patient is most likely located at which of the following spinal cord levels?
(A) C5
(B) C7
(C) T1
(D) T3
(E) T5
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Answer
The intrinsic muscles of the hand are primarily innervated by the C8 and T1 spinal nerves. Weakness and atrophy suggest lower motor neuron (LMN) involvement at or near the T1 level. Hyperreflexia and Increased Tone in the Lower Extremities are signs of an upper motor neuron (UMN) lesion. This occurs due to disruption of descending corticospinal tracts below the site of the lesion. A lesion at C5 and/or C7 would cause LMN signs in the biceps and deltoid muscles (innervated by C5-C6) rather than in the intrinsic hand muscles.
The correct answer is C