Which of the following diseases is X-linked?

• A. Hemophilia A
• B. Huntington disease
• C. Cystic fibrosis
• D. Polycystic kidney disease

Answer: (A)

X-linked inheritance is being tested here: a mutation on the X chromosome that typically affects males more often because they have only one X. Hemophilia A fits this pattern. It’s caused by a deficiency of clotting factor VIII and is inherited in an X-linked recessive way, so an affected male inherits the condition from a carrier mother, while females with one mutated X are usually carriers and often asymptomatic or only mildly affected.

The other diseases shown are not X-linked: Huntington disease is autosomal dominant, caused by a mutation in the HTT gene on an autosome; cystic fibrosis is autosomal recessive, requiring two mutated CFTR alleles; polycystic kidney disease is typically autosomal dominant, though there are rare recessive forms.