Concurrent phenylketonuria and immune-mediated central nervous system involvement: a case report highlighting rare co-occurrence

White-matter abnormalities are common in phenylketonuria (PKU) and are typically attributed to metabolic hypomyelination or intramyelinic edema. Immune-mediated CNS involvement co-occurring with PKU has rarely been described in adults. An 18-year-old male university freshman diagnosed with classical PKU presented with a 3-week history characterized by progressive bilateral lower-limb weakness and gait instability. Cervical spinal magnetic resonance imaging (MRI) revealed intramedullary lesions, indicating possible immune-mediated CNS involvement overlapping PKU. Cerebrospinal fluid (CSF) analysis was acellular but demonstrated positive oligoclonal IgG bands. Notable clinical and radiological improvement was observed following the administration of high-dose intravenous methylprednisolone. Considering the acute course, brain cord involvement, positive OCB, and steroid responsiveness, an acute immune-mediated CNS lesion overlapping PKU was diagnosed. The symmetric diffusion‑restricted brain lesions raised the possibility of concomitant PKU‑related intramyelinic edema. High‑dose intravenous methylprednisolone (1 g/day for 3 days) followed by an oral taper was administered, together with strict low‑phenylalanine diet and vitamin supplementation. Cognitive slowing and lower‑limb weakness improved substantially within 3 weeks. One‑month follow‑up MRI showed regression of spinal lesions and slight reduction of supratentorial white‑matter abnormalities. This case highlights a potential metabolic–immune overlap in PKU. Positive OCB and steroid‑responsive cord lesions support an inflammatory component, while symmetric diffusion‑restricted brain lesions suggest concurrent metabolic vulnerability. Longitudinal imaging and metabolic control (serial plasma phenylalanine/tyrosine, myelin‑sensitive MRI metrics) are essential to refine diagnosis and guide the need for disease‑modifying therapy.