Porphyria is a group of metabolic disorders rooted in heme, the iron-containing molecule in hemoglobin, which the body builds on an assembly line of eight enzymes split between the mitochondria and the cytosol. When a genetic flaw breaks one enzyme, half-finished porphyrins pile up and poison tissue. Acute hepatic porphyrias bring agonizing abdominal pain, racing heart, hallucinations and paralysis that doctors mistake for Guillain-Barré syndrome or lead poisoning, while cutaneous forms make sunlight burn and blister the skin. The name comes from the Greek porphyra, purple, after urine that darkens to a wine color in the sun.

In 1964 a British doctor linked the disease to werewolves, and in 1985 biochemist David Dolphin told the American Association for the Advancement of Science that porphyria explained vampires. Folklorists reject both ideas: 18th century vampires walked in daylight and looked plump and ruddy, and burning in the sun traces to the 1922 film Nosferatu. The episode then turns to King George III, whom Ida Macalpine and Richard Hunter diagnosed with acute intermittent porphyria in 1966, and the 2005 study that found high arsenic in his hair, likely from contaminated antimony in his own medicines.

• Why fasting, alcohol, hormones and common drugs such as barbiturates and sulfa antibiotics can trigger an attack
• How lead binds the same heme pathway enzymes and produces near identical symptoms
• Why ordinary chemical sunscreens fail when porphyrins react to visible blue light
• Theories tying porphyria to Mary Queen of Scots, Vincent van Gogh and Maria I of Portugal
• Treatments from intravenous dextrose and heme analogs to the RNA interference drug givosiran