Conflict of interest: The Institute of Mother and Child received a donation of oral sapropterin dihydrochloride (Dipharma, 100 mg tablets) from Almeda Pharmaceuticals AG (Dammstrasse 19, CH-6300 Zug, Switzerland; UID: CHE-349.606.441; Handelsregister-Nummer CH-170.3.038.129-6), intended for use in BH4 loading tests in patients with suspected BH4-responsive PKU. The donation was made on 24.12.2024, and the authors express their sincere gratitude for this support, which contributed to the completion of this work. This work was supported by a grant of the Ministry of Education and Research, CCCDI – UEFISCDI, project number PN-IV-PCB-RO-MD-2024-0539, within PNCDI IV