Primary Hyperoxaluria Type 1 AGXT Autosomal Recessive Pedigree

A 6-year-old boy presents with recurrent renal colic and passage of multiple stones since infancy. Imaging reveals bilateral nephrocalcinosis and radiopaque renal stones. Stone analysis confirms calcium oxalate monohydrate composition. 24-hour urine shows massive hyperoxaluria (>1.0 mmol/1.73m²/day) with elevated glycolate. His parents are first cousins and clinically unaffected. An older sibling died at age 12 with progressive kidney failure. Two other siblings are healthy. The inheritance pattern marked **C** in the pedigree diagram—showing affected siblings born to unaffected carrier parents with consanguinity and no vertical transmission—is most consistent with which genetic mechanism?