Nephropathic Cystinosis CTNS Autosomal Recessive Pedigree

An 18-month-old girl presents with failure to thrive, polyuria, polydipsia, and progressive muscle weakness. Examination reveals rachitic deformities, photophobia, and hazy corneas with pathognomonic fine glistening cystine crystals on slit-lamp examination. Laboratory studies show generalized proximal tubular dysfunction (Fanconi syndrome) with hypokalemia, hypophosphatemia, hyperchloremic metabolic acidosis, aminoaciduria, and glycosuria despite normal blood glucose. Leukocyte free-cystine is markedly elevated at 50–100 fold normal. The family pedigree shows **A: autosomal recessive inheritance with consanguineous carrier parents and two affected siblings**. Which of the following best explains the inheritance pattern and genetic basis of this condition?