Anatomy: USMLE Q No. 9

[Cardiac Arrest]

A 72-year-old man who is right handed suddenly develops paralysis in his right arm. He is alert, afebrile, and understands questions, but has great difficulty communicating his answers verbally. He has most likely had a stroke that involves the

A. left cerebral cortex.
B. right arcuate fasciculus.
C. right internal capsule.
D. right precentral gyrus.
E. superior temporal gyrus.

[dr-kenzy]

salam ..


ummm iam not sure ..
but i think the answer is :

A. left cerebral cortex.

waiting 4 the correct answer ..

thanx ..

[Cardiac Arrest]

Option A (left cerebral cortex) is correct. A lesion affecting the left cerebral cortex, including Brodmann areas 44 and 45, will disrupt Broca area (speech production area) and make it difficult for the patient to respond verbally. If it also affects Brodmann area 4 (the primary motor cortex), the lesion will also disrupt motor activity in the arm. Since many descending corticospinal fibers cross at the level of the pyramids, changes in motor activity would be expected in the contralateral (right) arm.

Option B (right arcuate fasciculus) is incorrect. The arcuate fasciculus runs between the Wernicke area (Brodmann area 22, which includes the auditory comprehension center) and the Broca area (Brodmann area 45, the speech production area). It does not include Brodmann area 4 (the primary motor cortex). Thus, a lesion in the arcuate fasciculus would affect the patient’s ability to respond verbally to questions, but would not affect motor activity in the arm.

Option C (right internal capsule) is incorrect. The internal capsule is a bundle of fibers that passes between the thalamus and the cortex. Damage to this structure would block sensory input from the midbrain, brainstem, and spinal cord, as well as motor output to those structures. Thus, damage to the right internal capsule would cause the left-sided paralysis seen in this patient. It would also cause cranial nerve damage and/or abnormal somatosensory activity, neither of which is apparent in this patient. Furthermore, it would not cause the communications problems experienced by this patient.

Option D (right precentral gyrus) is incorrect. Although a lesion in the right precentral gyrus (which contains the motor cortex) could cause left arm paralysis, it would not account for the patient’s communications problem.

Option E (superior temporal gyrus) is incorrect. The superior temporal gyrus contains the Wernicke area, which governs the interpretation of speech. Since the patient can understand questions asked of him, the Wernicke area may not be involved