**Core Concept**
Acquired haemophilia A is a rare autoimmune disorder characterized by the production of autoantibodies against factor VIII, leading to a deficiency of this clotting factor. This results in a prolonged activated partial thromboplastin time (APTT) while other coagulation tests remain normal.

**Why the Correct Answer is Right**
In acquired haemophilia A, the autoantibodies bind to and inactivate factor VIII, preventing it from participating in the coagulation cascade. This leads to a specific prolongation of the APTT, as factor VIII plays a crucial role in the intrinsic pathway of coagulation. The normal bleeding time and prothrombin time suggest that the problem lies in the intrinsic pathway, rather than the platelet count or the extrinsic pathway. Additionally, the normal platelet count rules out thrombocytopenia as a cause of the prolonged APTT.

**Why Each Wrong Option is Incorrect**

* **Option B:** Lupus anticoagulant is associated with a prolongation of the APTT, but it typically affects the prothrombin time as well, due to its effect on the phospholipid surface. In this case, the prothrombin time is normal.
* **Option C:** Disseminated intravascular coagulation (DIC) is characterized by a consumption of clotting factors and platelets, leading to a prolongation of both the APTT and prothrombin time, as well as thrombocytopenia. In this case, the platelet count is normal.
* **Option D:** Inherited congenital hemophilia A is caused by a deficiency of factor VIII, but it is present from birth and typically diagnosed in childhood. Acquired haemophilia A, on the other hand, typically presents in adults and is associated with a specific autoimmune response.

**Clinical Pearl / High-Yield Fact**
Acquired haemophilia A is a rare but potentially life-threatening condition that requires prompt recognition and treatment. It is essential to consider this diagnosis in any patient with a prolonged APTT and normal platelet count, especially in the postpartum period.

**✓ Correct Answer: A. Acquired haemophilia A**