I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
A 65-year-old woman with a history of rheumatic mitral valve disease is brought to the emergency department 30 minutes after the sudden onset of right-sided weakness and inability to speak. Neurologic examination shows weakness of the right lower side of the face and difficulty swallowing. Muscle strength is 3/5 on the right side. She can understand what is said to her, but she cannot repeat words or answer questions. An ECG shows atrial fibrillation. The most likely cause of the neurologic findings in this patient is occlusion of which of the following labeled arteries in the photograph of a normal brain?
(A) A
(B) B
(C) C
(D) D
(E) E
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Answer:
The patient’s symptoms of right-sided weakness, difficulty speaking, and difficulty swallowing suggest a stroke affecting the left hemisphere of the brain, as the left hemisphere is typically responsible for language and motor control of the right side of the body. Given her history of atrial fibrillation, she is at increased risk for embolic stroke.
Atrial fibrillation can lead to the formation of thrombi in the left atrium, which can then embolize and occlude cerebral arteries. The symptoms described—right-sided weakness, facial weakness, dysphagia, and expressive aphasia—point towards an occlusion of a major cerebral artery supplying the language and motor areas of the brain.
The artery most likely involved in this case is the left middle cerebral artery (MCA). The MCA supplies a significant portion of the lateral cerebral cortex, including areas involved in motor function (precentral gyrus), language production (Broca’s area in the dominant hemisphere, typically the left), and sensory function.
The correct answer is D
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