**Core Concept**
Hemophilia A is a genetic disorder caused by a deficiency in a specific protein essential for blood clotting, leading to prolonged bleeding episodes. This protein is a crucial component of the coagulation cascade, specifically involved in the intrinsic pathway.

**Why the Correct Answer is Right**
The correct answer is related to the deficiency of Factor VIII, a key protein in the intrinsic pathway of blood coagulation. Factor VIII serves as a cofactor for Factor IXa in the activation of Factor X, a critical step in the formation of a blood clot. Without sufficient Factor VIII, the coagulation cascade is impaired, resulting in the inability to form a proper blood clot, leading to bleeding episodes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because Hemophilia B (also known as Christmas disease) is caused by a deficiency of Factor IX, not Factor VIII.
**Option B:** This option is incorrect because Hemophilia A is not caused by a deficiency of platelets; it is a coagulation factor disorder.
**Option C:** This option is incorrect because von Willebrand disease is a separate bleeding disorder that affects the von Willebrand factor, which is involved in platelet adhesion and coagulation factor VIII transport.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Hemophilia A and B are X-linked recessive disorders, meaning they predominantly affect males, who have only one X chromosome. Females can be carriers of these disorders but usually do not exhibit symptoms.

**Correct Answer:** A. Deficiency of Factor VIII.