C

Homocystinuria and classic galactosemia frequently cause neuro-cognitive sequelae; untreated patients often develop intellectual disability, whereas alkaptonuria and pentosuria are largely benign with respect to cognition. [Rais, 2003, 14691398] describes mental retardation in several siblings with homocystinuria, and [Lo, 1984, 6701054] reports siblings with well-treated galactosemia who nonetheless developed progressive mental retardation. Therefore, options b and d are the correct choices.