Von willebrands disease all are true except -a) Factor VIII C deficiencyb) BT Prolongedc) Normal Ristocetin testd) Defective aggregation

**Core Concept**
Von Willebrand disease (VWD) is a congenital bleeding disorder caused by a deficiency or dysfunction of von Willebrand factor (VWF), a crucial protein involved in platelet adhesion and blood coagulation. VWF plays a key role in stabilizing factor VIII and facilitating its binding to platelets.

**Why the Correct Answer is Right**
Option **c) Normal Ristocetin test** is incorrect because the Ristocetin cofactor assay is used to diagnose VWD by assessing the ability of VWF to facilitate platelet agglutination in response to ristocetin. In VWD, this response is impaired. A normal Ristocetin test would suggest that VWF is functioning normally, which is not the case in VWD. The correct answer involves abnormalities in VWF function, such as **d) Defective aggregation**, which is a hallmark of VWD.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor VIII C deficiency is incorrect because VWD is characterized by a deficiency or dysfunction of VWF, not factor VIII C. Factor VIII C deficiency is a characteristic of Hemophilia A, a different bleeding disorder.
**Option B:** BT Prolonged is correct, as bleeding times (BT) are often prolonged in patients with VWD due to impaired platelet adhesion.
**Option D:** Defective aggregation is correct, as it accurately describes the impaired platelet aggregation seen in VWD.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that VWD can be caused by either a deficiency of VWF (Type 1) or a qualitative defect in VWF (Type 2 or 3), and that bleeding time and ristocetin cofactor assays can help differentiate between these subtypes.

**Correct Answer: D. Defective aggregation**