Question

test if the substantia nigra is functional in those suspected of parkinson’s, look for the levels his precursor:
serotonin
dopamine
phosphodiesterase
melatonin

Explanation:

Parkinson's disease is associated with the degeneration of dopaminergic neurons in the substantia nigra, which produce dopamine. To test the functionality of the substantia nigra in suspected Parkinson's cases, we look at dopamine levels (or its precursor - though here the options have dopamine itself as a key marker related to substantia nigra function). Serotonin is related to mood/other systems, phosphodiesterase is an enzyme, and melatonin is for circadian rhythm, so dopamine is the correct choice.

Answer:

dopamine