Sanfilippo Syndrome

A 4-year-old girl presents to pediatric genetics with developmental regression, severe behavioral disturbance, and progressive loss of language skills that began after age 2 years. Her 2-year-old sister is currently asymptomatic but molecular testing confirms she carries the same biallelic pathogenic variants. Both parents are phenotypically normal and heterozygous carriers. Enzyme assay shows markedly deficient heparan-N-sulfatase (SGSH) activity, confirming Sanfilippo syndrome type A. A three-generation pedigree is constructed showing affected females in the same sibship, unaffected parents, and no sex-linked or maternal-only pattern. The inheritance pattern marked **A** in the diagram is most consistent with which of the following?