BioMarin (NASDAQ: BMRN), a massively successful rare-disease drugmaker that's already won six FDA approvals, is expected to find out this week if the FDA will gives it a seventh victory. The regulatory decision on pegvaliase, a new treatment for phenylketonuria, or PKU, has big implications for the company and its investors, so let's take a closer look.PKU is a rare genetic disease caused by an inability to break down phenylalanine, an amino acid.