Correct Answer » C Explanation

Achondroplasia is an autosomal dominant (AD) disorder caused by a mutation in the FGFR3 (Fibroblast Growth Factor Receptor 3) gene on chromosome 4.

Key features include:

• Most common cause of disproportionate dwarfism.
• Short limbs with a relatively normal trunk.
• Macrocephaly with frontal bossing.
• Midface hypoplasia.
• Most cases arise due to new (de novo) mutations, with increasing paternal age being a risk factor.

Therefore, Achondroplasia is the exception, as it is inherited in an autosomal dominant pattern.

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Why Other Options Are Incorrect

A. Phenylketonuria (PKU)

• Incorrect because Phenylketonuria is an autosomal recessive disorder.
• Caused by deficiency of phenylalanine hydroxylase, leading to accumulation of phenylalanine.
• Features include intellectual disability, seizures, fair skin, eczema, and a musty body odor if untreated.

B. Galactosaemia

• Incorrect because Galactosaemia is inherited as an autosomal recessive disorder.
• Most commonly due to deficiency of galactose-1-phosphate uridyltransferase (GALT).
• Presents with jaundice, hepatomegaly, vomiting, cataracts, and risk of Escherichia coli sepsis.

D. Wilson’s Disease

• Incorrect because Wilson’s disease is an autosomal recessive disorder.
• Caused by mutation in the ATP7B gene resulting in impaired copper metabolism.
• Characterized by hepatic disease, neurological manifestations, psychiatric symptoms, and Kayser–Fleischer rings.