**Core Concept**
Hemophilia A is a genetic disorder caused by a deficiency in factor VIII, a crucial protein required for blood clotting. This deficiency leads to prolonged bleeding, especially after injuries or surgeries. The condition is usually inherited in an X-linked recessive pattern, predominantly affecting males.

**Why the Correct Answer is Right**
Factor VIII is a vitamin K-dependent serine protease that plays a central role in the intrinsic pathway of blood coagulation. It forms a complex with factor IXa, factor X, and factor VIIIa to activate factor X, leading to the formation of thrombin and subsequent clot formation. In hemophilia A, the lack of functional factor VIII impairs this process, resulting in impaired clot formation. The diagnosis of hemophilia A is typically confirmed through coagulation studies, including the partial thromboplastin time (aPTT) and factor VIII activity assays.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because hemophilia A is indeed caused by a deficiency in factor VIII, not factor IX. While both factors are involved in the intrinsic pathway, they play distinct roles, with factor VIII being the primary target in hemophilia A.
**Option B:** Incorrect because the aPTT is a commonly used test to diagnose hemophilia A, as it measures the time it takes for blood to clot in the presence of tissue factor. Prolonged aPTT in hemophilia A is due to the deficiency of factor VIII.
**Option C:** Incorrect because hemophilia A is primarily inherited in an X-linked recessive pattern, which means the mutated gene responsible for the condition is located on the X chromosome. Males, who have only one X chromosome, are more likely to be affected, while females can be carriers.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the diagnosis of hemophilia A is often confirmed through a combination of clinical history, physical examination, and laboratory tests, including coagulation studies and factor VIII activity assays. A thorough understanding of the coagulation cascade and the role of factor VIII in hemophilia A is crucial for accurate diagnosis and management.

**Correct Answer: A. Hemophilia A is caused by a deficiency in factor IX.**