**Core Concept**
Haemophilia A is a genetic disorder characterized by a deficiency in factor VIII, a crucial coagulation factor necessary for the intrinsic pathway of blood coagulation. This deficiency leads to impaired clot formation, resulting in spontaneous or traumatic bleeding.

**Why the Correct Answer is Right**
The correct answer is due to the deficiency of factor VIII, which is essential for the activation of factor X by the factor VIII-factor IX complex. This complex is formed when factor VIII binds to the phospholipid surface and factor IXa. The resulting factor X activation then leads to the formation of thrombin, which converts fibrinogen to fibrin, forming a stable clot. Without sufficient factor VIII, this process is severely impaired.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the primary issue in haemophilia A is not related to factor IX, which is the coagulation factor deficient in haemophilia B (also known as Christmas disease).
* **Option B:** This option is incorrect because haemophilia A is not caused by a deficiency in fibrinogen, which is necessary for clot formation but not directly involved in the intrinsic pathway.
* **Option C:** This option is incorrect because the primary issue in haemophilia A is not related to the activation of factor X by tissue factor, which is the primary pathway in the extrinsic coagulation cascade.

**Clinical Pearl / High-Yield Fact**
Haemophilia A is an X-linked recessive disorder, meaning it primarily affects males, who have only one X chromosome. Females can be carriers, but they typically do not exhibit symptoms due to their second X chromosome.

**Correct Answer: D. Deficiency of factor VIII.**