Which of the following is the BEST reason why Parkinson's disease cannot be treated by simply administering dopamine into the blood stream:

W) dopamine's blood half-life is too short

X) dopamine will not cross the blood brain barrier

Y) dopamine is the precursor to the real effective transmitter in Parkinson's disease

Z) it is not known why dopamine will not work if administered directly into the blood
X) DOPAMINE WILL NOT CROSS THE BLOOD BRAIN BARRIER