What vitamin does the body make itself?

Vitamin D -

All others must be derived from the diet.

Right , and wrong at the same time. The body can naturally produce more than 1 vitamin.

Using cholesterol and UVB rays from the sun, the body can produce 10,000 IU of vitamin D from 30 mins of optimum sun exposure ( large areas of the skin exposed with NO sunscreen. )

The body can also produce Vitamin B3 (Niacin) using the amino acid Tryptophan. 1 mg of Niacin is produced for every 60 mg of Tryptophan consumed.

Recently there has been research into vitamins produced in the gut. Certain beneficial bacteria have been shown to produce B vitamins and vitamin K .

This is just off of the top of my head. There are probably more.....