## **Core Concept**
The patient's presentation suggests a bleeding disorder, likely **Hemophilia A**, given the low serum level of **clotting factor VIII** (<2% of normal). This condition is characterized by a deficiency in factor VIII, a crucial component in the coagulation cascade. The patient's symptoms, including easy bruising, acute onset of flank pain with ecchymosis after physical exercise, and a family history of similar symptoms, are indicative of a hereditary coagulation disorder. ## **Why the Correct Answer is Right** The correct answer, **X-linked recessive inheritance**, is the most likely underlying mechanism for the bleeding tendency in this patient. Hemophilia A is caused by a deficiency in factor VIII, which is inherited in an **X-linked recessive pattern**. This means the gene responsible for the condition is located on the **X chromosome**. Males have one X and one Y chromosome (XY), and females have two X chromosomes (XX). In the case of Hemophilia A, males are more frequently affected because they have only one X chromosome. If that X chromosome has the mutation for Hemophilia A, they will express the condition since they do not have another X chromosome to compensate. Females can be carriers, having one normal and one mutated X chromosome, and they have a 50% chance of passing the mutated gene to each son, who would be affected, and to each daughter, who would typically be a carrier. ## **Why Each Wrong Option is Incorrect** - **Option A:** Autosomal dominant inheritance is incorrect because conditions inherited in this pattern would affect both males and females equally, and a single copy of the mutated gene is enough to cause the condition. The family history and gender distribution do not fit well with this pattern for Hemophilia A. - **Option B:** Autosomal recessive inheritance is incorrect because, while it could result in a similar presentation, it would not explain the **X-linked** pattern of inheritance seen with Hemophilia A, nor the gender disparity in affected individuals. - **Option D:** Mitochondrial inheritance is incorrect because mitochondrial DNA disorders are inherited solely from mothers and affect both genders equally. They also do not follow the specific pattern of inheritance seen in this patient's family. ## **Clinical Pearl / High-Yield Fact** A key clinical pearl is that **Hemophilia A and B** (another coagulation factor deficiency) are **X-linked recessive disorders**. Affected individuals often present with **joint or muscle bleeds** and have a history of **easy bruising**. Family history often reveals other affected males and female carriers. Knowing that **desmopressin** can be used to manage mild cases of Hemophilia A by increasing factor VIII levels is also crucial. ## **Correct Answer:** . **X-linked recessive inheritance**