<s>[INST] context: The thyroid hormones, triiodothyronine (T3 ) and thyroxine (T4 ), are produced in the thyroid gland from tyrosine residuesin thyroglobulin.Parkinson disease is caused by low levels of dopamine in the nervous system. In the initial stages of Parkinson disease, giving DOPA can reduce the severity of the symptoms because DOPA can be decarboxylated to form dopamine. DOPA can easily enter the brain, whereas dopamine cannot. A defect in the synthesis or processing of collagen will lead to a variety of diseases, of which Ehlers-Danlos syndrome is one (osteogenesis imperfecta is another). Tay-Sachs disease results from an inability to degrade GM2, a ganglioside (it is also a sphingolipid). The enzyme missing is b-hexosaminidase. McArdle disease is caused by a defective muscle glycogen phosphorylase, such that the muscle cannot generate glucose from glycogen, leading to exercise intolerance. Maple syrup urine disease is caused by the lack of branched- chain a-keto acid dehydrogenase activity, a necessary step in the metabolism of the branched-chain amino acids (leucine, isoleucine, and valine). [/INST] question: Parkinson disease can be caused by a problem with the metabolism of which particular compound. | options: A: Glycogen B: Collagen C: Dopamine D: Valine | answer: C </s>