Nicholas Brendon’s cause of death has been revealed. The “Buffy the Vampire Slayer” star — who died on March 20 in Indiana — died of atherosclerotic and hypertensive cardiovascular disease, with acute pneumonia and previous myocardial infarction (heart attack) being contributing factors, according to his autopsy report obtained exclusively by Page Six. Arteriosclerosis occurs when the blood vessels that carry oxygen and nutrients from the heart to the rest of the body — the arteries — become thick and stiff, hardening over time. Meanwhile, acute pneumonia is an intense, sudden-onset infection that causes inflammation in the lungs due to fluid or pus filling the air sacs. The mechanism of Brendon’s death, which was determined to be natural, was 90% blockage of his right coronary artery. Nicholas Bendon’s cause of death has been revealed. He’s seen here in a previous Facebook video. The “Buffy the Vampire Slayer” star, pictured above in a 2000 promo shot for the series, died on March 20 in Indiana.…