Common Single-Gene Disorders

A 3-year-old boy from North India presents with progressive muscle weakness, difficulty climbing stairs, and frequent falls. On examination, he has pseudohypertrophy of the calf muscles, a waddling gait, and positive Gowers' sign. Serum creatine kinase (CK) is markedly elevated at 8,500 IU/L. Genetic testing reveals a deletion in the dystrophin gene on the X chromosome. What is the most likely diagnosis?

A 6-month-old girl born to consanguineous parents presents with poor feeding, developmental delay, and a distinctive musty or 'mousy' odor to her urine and sweat. Newborn screening was not performed. On examination, she has fair skin, light-colored hair, and eczematous rash. Serum phenylalanine level is 1,200 μmol/L (normal <120 μmol/L). What is the most likely diagnosis?

Which enzyme deficiency causes maple syrup urine disease (MSUD)?

In familial hypercholesterolemia (FH), which gene mutation is most commonly responsible for the autosomal dominant form?