**Core Concept**
Von Willebrand disease (VWD) is a bleeding disorder caused by a deficiency or dysfunction of von Willebrand factor (VWF), a protein essential for platelet adhesion and blood coagulation. It is the most common hereditary coagulation abnormality.

**Why the Correct Answer is Right**
Von Willebrand disease is characterized by a deficiency or qualitative defect in von Willebrand factor, which leads to impaired platelet adhesion and aggregation. The disease is typically associated with **defective aggregation** of platelets due to the inability of VWF to bind to platelet surface receptors. This results in prolonged bleeding times and an increased risk of bleeding. **Factor VIII C deficiency** is actually associated with Hemophilia A, not Von Willebrand disease. **Bleeding Time (B.T.) Prolonged** is a characteristic feature of VWD, as the disease impairs platelet adhesion and aggregation. **Normal Ristocetin test** is incorrect because the Ristocetin cofactor assay is used to assess the activity of VWF, and in VWD, this test is often abnormal.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor VIII C deficiency is a characteristic of Hemophilia A, not Von Willebrand disease. Hemophilia A is caused by a deficiency of Factor VIII, which is a different protein from VWF.
**Option C:** The Ristocetin test is used to assess the activity of VWF, and in VWD, this test is often abnormal due to the defective or deficient VWF.

**Clinical Pearl / High-Yield Fact**
In Von Willebrand disease, the bleeding time is often prolonged due to impaired platelet adhesion and aggregation. This can be a key distinguishing feature between VWD and other bleeding disorders.

**Correct Answer:** D. Defective aggregation