**Core Concept**
Hemophilia A is a genetic disorder characterized by a deficiency of a specific clotting factor, leading to prolonged bleeding and a susceptibility to bleeding disorders.

**Why the Correct Answer is Right**
Hemophilia A, also known as classic hemophilia, is caused by a deficiency of factor VIII, a crucial protein that helps blood to clot. Factor VIII is a carrier protein for factor IX, which is involved in the intrinsic pathway of blood coagulation. Without sufficient factor VIII, the blood's ability to form clots is severely impaired, leading to the characteristic bleeding symptoms of hemophilia A.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor II is involved in the extrinsic pathway of coagulation, not the intrinsic pathway affected in hemophilia A.
**Option B:** Factor X is a common pathway factor that is involved in both the intrinsic and extrinsic pathways, but its deficiency leads to a different bleeding disorder.
**Option C:** Factor XIII is involved in stabilizing the blood clot, but its deficiency does not cause hemophilia A.

**Clinical Pearl / High-Yield Fact**
Hemophilia A is an X-linked recessive disorder, meaning it primarily affects males, who have only one X chromosome. Females can be carriers of the mutated gene, but they are usually asymptomatic or have mild symptoms.

**Correct Answer:** B. Factor VIII.