Maple Syrup Urine Disease

A 6-day-old term male neonate born to Old Order Mennonite parents presents to the NICU with poor feeding, lethargy, and a characteristic sweet, maple-syrup-like odor of urine. Newborn screening shows markedly elevated leucine (2,800 micromol/L), elevated isoleucine and valine, and pathognomonic allo-isoleucine. Brain MRI reveals diffuse white matter edema affecting cerebellar white matter and posterior limb of internal capsule. BCKDHA gene sequencing identifies homozygosity for a founder mutation. The pedigree shows affected males and females with unaffected parents, multiple affected cousins from consanguineous unions, and no vertical transmission. The inheritance pattern marked **A** in the diagram is most consistent with which of the following?