**Question:** Which of the following is X-linked recessive disorder?

A. Duchenne muscular dystrophy
B. Hemophilia
C. Cystic Fibrosis
D. Marfan syndrome

**Core Concept:**
X-linked recessive disorders are genetic conditions that are inherited from an affected parent to their children through the X chromosome. In this inheritance pattern, the affected parent (carrier) has one normal X chromosome and one mutated X chromosome, while the unaffected parent has two normal X chromosomes. The offspring inherit the X chromosomes from their parents, resulting in four possible combinations:

1. Carrier (both parents are carriers): Neither child will have the condition.
2. Affected (male), Carrier (female): The male will have the X-linked recessive disorder, while the female will be a carrier.
3. Carrier (male), Affected (female): The female will have the X-linked recessive disorder, while the male will be a carrier.
4. Carrier (both parents are carriers): Both children will be carriers.

**Why the Correct Answer is Right:**
Hemophilia is the correct answer because it is an X-linked recessive disorder. In this case, the affected person is a male (XY) who inherited the mutated X chromosome from his carrier mother, who has one normal X chromosome (XX) and one mutated X chromosome (Xm). The carrier mother does not show symptoms because she has two normal X chromosomes. Hemophilia A is caused by a deficiency of clotting factor VIII, while Hemophilia B is caused by a deficiency of clotting factor IX. Both Hemophilia A and B are inherited through the X-linked recessive pattern.

**Why Each Wrong Option is Incorrect:**

A. Duchenne muscular dystrophy: This is a X-linked dominant disorder, meaning that if a male inherits the mutated gene, he will have the condition (XY), and females with two X chromosomes (XX) are usually carriers but can also be affected.

B. Marfan syndrome: This is a complex autosomal dominant disorder involving connective tissue abnormalities. It affects both males and females, so it does not fit the X-linked recessive inheritance pattern.

C. Cystic fibrosis: This is an autosomal recessive disorder, meaning it is inherited in an autosomal recessive pattern (XY, XX), not X-linked recessive pattern.

D. Sickle cell anemia: This is an autosomal recessive disorder, inherited through autosomal genes (XY, XX).

**Clinical Pearl:**

Understanding the inheritance patterns of genetic disorders is essential for diagnosing and managing patients. If a male presents with a bleeding disorder, one should consider X-linked recessive disorders like Hemophilia A and B. In females, if they present with bleeding or bleeding-related symptoms, they could be carriers or could have autosomal recessive disorders like Hemophilia A and B. If the patient is a male, the presence of severe joint pain and deformed joints could indicate Hemophilia A or B, which affect the blood clotting factors