I am going to answer the sample USMLE Step 1 Questions with Explanation. These questions are available at USMLE.org
Q- A 56-year-old man is brought to the emergency department by his wife 30 minutes after he had severe upper back pain and hoarseness before becoming comatose. He has hypertension treated with hydrochlorothiazide. His temperature is 37°C (98.6°F), pulse is 100/min, and blood pressure is 160/80 mm Hg in the right arm and 100/60 mm Hg in the left arm. Ophthalmologic examination shows ptosis and miosis of the left eye. There is anhidrosis of the left side of the forehead, right hemiplegia, and a decreased left radial pulse. A chest x-ray shows a widened mediastinum. Which of the following conditions is the most likely cause of these findings?
(A) Dissection of the aorta distal to the left subclavian artery
(B) Dissection of the aorta extending into the left carotid artery
(C) Dissection of the aorta extending into the left carotid artery and distal aortic arch
(D) Dissection of the proximal aorta extending into the right subclavian artery
(E) Superior sulcus tumor
(F) Thrombus of the left carotid artery
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Answer
This is a typical case of acute aortic dissection involving the ascending aorta with extension into the left carotid artery and the distal aortic arch. Severe upper back pain and sudden onset of symptoms suggest acute aortic dissection. Blood pressure which is higher in the right arm compared to the left arm and decreased left radial pulse suggest the involvement of the aortic arch. Right hemiplegia indicates reduced blood flow to the left side of the brain which occurs due to dissection affecting the left carotid artery.
The correct answer is C