Retinaldehyde
Retinaldehyde also known as retinal is a vitamin A derivative that is a pre-cursor to retinoic acid. Retinaldehyde not to be confused with retinol — is the strongest of the over-the-counter retinoid. One step closer to retinoic acid on the conversion scale, it only takes one step for it to convert and become active, making it more potent than the aforementioned retinyl esters and retinol.