Which vitamin deficiency causes scurvy?

Vitamin C, and the story of learning that is a case study in how knowledge gets lost. James Lind ran one of the first controlled clinical trials in 1747, giving sick sailors different treatments and finding that citrus worked - and it took the Royal Navy over forty years to adopt lemon juice as policy.

Then it was unlearned. The navy switched from Mediterranean lemons to cheaper West Indian limes, which have less vitamin C, and boiled the juice in copper vessels, which destroys more - so scurvy reappeared and the theory was discredited. By the time of the Antarctic expeditions of the early 1900s, the leading explanation was ptomaine poisoning from tainted tinned meat, and Scott's men suffered accordingly.

Most animals make their own. Humans, other primates, guinea pigs and a few others carry a broken copy of the gene for the final enzyme - it is still in the genome, disabled - so we must eat it, and we get scurvy when we do not. That is why guinea pigs were the experimental animal that let vitamin C be isolated in the 1930s: they share the same fault.