**Question:** Which of the following has X linked recessive inheritance?

A. Duchenne muscular dystrophy
B. Achondroplasia
C. Hemophilia A
D. Cystic Fibrosis

**Core Concept:** X-linked recessive inheritance is a mode of inheritance where a gene for a trait is located on the X chromosome. In this type of inheritance, a male (XY) needs to inherit two mutated copies (one from each parent) to be affected, while a female (XX) only needs one mutated copy to be a carrier.

**Why the Correct Answer is Right:**

C. Hemophilia A falls into the category of X-linked recessive inheritance. In this case, the mutated gene is responsible for producing a clotting factor VIII (FVIII), which is crucial for blood clotting. Hemophilia A affects males (XY) because they have only one X chromosome. If a male inherits a mutated FVIII gene from his mother, he will be a carrier, but if he receives a mutated gene from both parents, he will develop Hemophilia A, with severe bleeding episodes due to the lack of functional FVIII.

**Why Each Wrong Option is Incorrect:**

A. Duchenne muscular dystrophy is a X-linked dominant disorder, meaning that a male with one mutated gene (XY) will be affected, while a female with one mutated gene (XY) will be a carrier.

B. Achondroplasia is a type of dwarfism caused by a mutation in the FGFR3 gene, which is located on chromosome 4, not X. Achondroplasia primarily affects males and females equally due to autosomal dominant inheritance.

D. Cystic Fibrosis is also an X-linked recessive disorder, but it is caused by mutations in the CFTR gene, which is located on chromosome 7, not X. Cystic fibrosis affects both males and females at a similar rate due to X-linked recessive inheritance.

**Clinical Pearl:**

Understanding the mode of inheritance is crucial when evaluating patients with specific disorders and their family history. This knowledge helps in predicting the sex of an affected individual and the risk of carrier status for females.