**Core Concept**
Hemophilia is a bleeding disorder characterized by the inability of blood to clot properly, leading to prolonged bleeding. It is caused by a deficiency of specific coagulation factors, which are proteins essential for the blood clotting process.

**Why the Correct Answer is Right**
Hemophilia A is caused by a deficiency of factor VIII, a protein that plays a crucial role in the intrinsic pathway of blood coagulation. Factor VIII acts as a cofactor for factor IXa in the activation of factor X to factor Xa, which then catalyzes the conversion of prothrombin to thrombin. Without sufficient factor VIII, the blood cannot clot properly, leading to the characteristic symptoms of hemophilia A. The deficiency of factor VIII in hemophilia A is usually inherited in an X-linked recessive pattern, affecting mostly males.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor I (Fibrinogen) - Fibrinogen is a glycoprotein that is essential for the formation of fibrin clots, but its deficiency leads to afibrinogenemia, a different bleeding disorder. Hemophilia A is not caused by a deficiency of fibrinogen.
**Option B:** Factor XIII - Factor XIII is a transglutaminase that stabilizes the fibrin clot by cross-linking fibrin molecules. Its deficiency leads to afibrinogenemia, a different bleeding disorder. Hemophilia A is not caused by a deficiency of factor XIII.
**Option C:** Factor VII - Factor VII is a vitamin K-dependent coagulation factor that initiates the extrinsic pathway of blood coagulation. Its deficiency leads to bleeding disorders, but hemophilia A is not caused by a deficiency of factor VII.

**Clinical Pearl / High-Yield Fact**
Hemophilia A is more common than hemophilia B (due to factor IX deficiency) and accounts for about 80-85% of all hemophilia cases.

**Correct Answer: D. Factor VIII**