What is the most likely diagnosis?

A 62-year-old man with a history of poorly controlled hypertension comes to the
emergency department complaining of 1 hour of intense pain in his chest. He was
climbing the stairs at home when he felt a sudden, sharp pain in the center of his
chest, and felt light-headed. He still feels light-headed and reports that the pain is a
“stabbing,” 10 out of 10 pain that radiates throughout his chest. He denies any other
medical history, but on further questioning reveals that he smokes one pack of cigarettes
daily and takes his metoprolol only when he feels “sick” or has a headache. Vital signs
include a temperature of 37.0°C (98.6°F), blood pressure of 170/100 mm Hg, pulse
rate of 85/min, and respiratory rate of 15/min. Physical examination shows a welldeveloped,
well-nourished, uncomfortable man with slight diaphoresis. His HEENT
and neck examinations are unremarkable. His lung fi elds are clear to auscultation and
he has good breath sounds bilaterally. His heart has a regular rate and rhythm with
a 2/6 diastolic murmur along the right sternal border. Examination of his extremities
reveals an absent right radial pulse. His abdominal and neurologic examinations are
unremarkable. X-ray of the chest shows clear lung fi elds and a widened mediastinum.