A 19-year-old man is having recurrent bleeding occur in his knee when playing contact sports. He has no history of spontaneous bleeding, but his brother has similar problems. Consultation with a specialist reveals that he has “mild” hemophilia A. Which of the following factor abnormalities is consistent with this diagnosis?

**Core Concept**
Hemophilia A is a genetic disorder caused by a deficiency in factor VIII, a crucial protein in the coagulation cascade. This deficiency leads to impaired platelet plug formation and prolonged bleeding, especially in joints and muscles.

**Why the Correct Answer is Right**
The correct answer is consistent with hemophilia A because factor VIII plays a pivotal role in the intrinsic pathway of coagulation. It forms a complex with factor IXa and activates factor X, leading to the conversion of prothrombin to thrombin and subsequent clot formation. In individuals with hemophilia A, the reduced levels of factor VIII result in impaired coagulation, manifesting as recurrent joint bleeding and a positive family history. This is in line with the presentation of the 19-year-old man and his brother.

**Why Each Wrong Option is Incorrect**
* **Option A:** Factor I (fibrinogen) is a key component in the coagulation cascade, but its deficiency leads to dysfibrinogenemia, a different bleeding disorder characterized by abnormal fibrin clot formation.
* **Option B:** Factor XIII is involved in stabilizing fibrin clots, but its deficiency is associated with a bleeding disorder known as fibrin stabilization disorder, which presents with easy bruising and prolonged bleeding.
* **Option D:** Factor XI is involved in the intrinsic pathway of coagulation, but its deficiency is associated with hemophilia C, a less common bleeding disorder characterized by a milder bleeding phenotype compared to hemophilia A.

**Clinical Pearl / High-Yield Fact**
It is essential to note that hemophilia A is X-linked recessive, meaning it primarily affects males. Female carriers can exhibit mild symptoms or remain asymptomatic due to X-chromosome inactivation.

**Correct Answer:** C. Factor VIII deficiency is consistent with this diagnosis.

✓ Correct Answer: B. decreased levels of functional factor VIII