A 68-year-old hypertensive man presents with acute chest pain, hypotension (60/40 mm Hg), muffled heart sounds, raised JVP, and pulsus paradoxus of 20 mm Hg on day 6 of admission for anterior ST-elevation MI. Echocardiography confirms acute cardiac tamponade. At autopsy, the pericardial sac is tensely distended with approximately 350 mL of dark-red clotted blood, as marked **A** in the diagram. Dissection reveals a 1.5 cm ragged full-thickness tear of the anterior left ventricular free wall. Which of the following is the most common non-traumatic cause of the pathology marked **A**?