Final answer:
The most likely diagnosis for the patient presenting with a history of hypertension, chest pain radiating to the back, signs of left ventricular hypertrophy, and widening of the superior mediastinum on a chest roentgenogram is Aortic dissection.
Step-by-step explanation:
A patient with a history of hypertension, chest pain radiating to the back, an electrocardiogram showing left ventricular hypertrophy, and a chest roentgenogram revealing a widening of the superior mediastinum likely has a diagnosis of Aortic dissection. This condition is a medical emergency characterized by a tear in the inner layer of the large blood vessel branching off the heart (the aorta), allowing blood to flow between the layers of the blood vessel wall, causing separation (dissection) and potentially leading to rupture. Other options like acute severe aortic insufficiency, mitral valve prolapse, and pericarditis do not typically present with the combination of symptoms and findings described, making them less likely in this scenario.