Q. A 36-year-old man is seen in the clinic for a routine check-up. As a medical student accompanying the physician, you learn that the patient has a long-standing history of depression, which has been challenging to manage with standard therapies. The physician discusses considering a new approach that might involve the administration of tyrosine, an amino acid. You recall from your medical school biochemistry course that tyrosine is an important precursor for several vital biomolecules, including the neurotransmitter dopamine. Conversion of tyrosine to DOPA is a crucial step in dopamine synthesis. Which of the following is a necessary cofactor in the conversion of tyrosine to DOPA?