Congenital Myasthenic Syndrome (CHRNE / Postsynaptic AChR Deficiency) - Autosomal Recessive Pedigree

A 6-year-old boy from a consanguineous Sindhi family presents with bilateral ptosis since birth, restricted eye movements, fatigable weakness worsening through the day, and delayed motor milestones. His 11-year-old sister has mild ptosis and fatigability, his 9-year-old brother is unaffected, and his 4-year-old sister has prominent ptosis. Both parents are clinically normal. Serum acetylcholine receptor and MuSK antibodies are negative. Repetitive nerve stimulation shows >15% decremental response. The pedigree pattern shown in the diagram is marked as **C**. Which inheritance pattern does this pedigree most accurately represent, and what is the underlying genetic basis of this patient's condition?