The Amino Acid, Tyrosine, Is the First Precursor in the Biosynthetic

Multiple Choice

The amino acid, tyrosine, is the first precursor in the biosynthetic pathway that ultimately produces dopamine, which has been drastically decreased in Parkinson's Disease. Why cannot one simply consume large amounts of tyrosine to make up for the loss?

A) The amount of tyrosine needed to produce enough dopamine is toxic.
B) Tyrosine does not easily cross the blood brain barrier.
C) Tyrosine is converted into something else in the periphery before it reaches the CNS.
D) The neurons that convert tyrosine into dopamine have been destroyed.
E) It is impossible to target tyrosine directly to the neurons that produce dopamine from tyrosine.