Q. A physician is treating a 60-year-old individual who has recently been diagnosed with Parkinson's disease. In discussing prospective treatments, she explains that a frequently adopted strategy is to give the patient L-DOPA, a compound that acts as a precursor to dopamine. This compound is unique in its ability to cross the blood-brain barrier, and once it does, it transforms into dopamine within the brain. She then queries a medical student, inquiring if he recalls the enzyme that facilitates this transformation. Which of the following is the correct enzyme?