MONDO_0009862 (dihydropteridine reductase deficiency) can be described as follows. Dihydropteridine reductase (DHPR) deficiency is a severe form of hyperphenylalaninemia (HPA) due to impaired regeneration of tetrahydrobiopterin (BH4), leading to decreased levels of neurotransmitters (dopamine, serotonin) and folate in cerebrospinal fluid, and causing neurological symptoms such as psychomotor delay, hypotonia, seizures, abnormal movements, hypersalivation, and swallowing difficulties. Also known as: 6,7-dihydropteridine reductase activity disease, PKU type 2, disorder of 6,7-dihydropteridine reductase activity, hyperphenylalaninemia due to dihydropteridine reductase deficiency, hyperphenylalaninemia, BH4-deficient C, hyperphenylalaninemia, Bh4-deficient, type C, phenylketonuria type 2.