**Core Concept**
The patient's symptoms of prolonged bleeding, easy bruising, and prolonged bleeding time indicate a bleeding disorder. The reduced factor VIII level and ristocetin cofactor assay suggest a deficiency in von Willebrand factor (VWF), a protein essential for platelet adhesion and aggregation.

**Why the Correct Answer is Right**
Von Willebrand disease (VWD) is a genetic disorder characterized by a deficiency or dysfunction of VWF, leading to impaired platelet adhesion and aggregation. The ristocetin cofactor assay is a specific test for VWF function. Factor VIII is also affected in VWD because VWF serves as a carrier protein for factor VIII in the circulation. The patient's normal PT and PTT suggest the absence of a coagulation factor deficiency affecting the intrinsic or common pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** Factor XI deficiency is a bleeding disorder characterized by prolonged PTT, which is not present in this patient.
**Option B:** Hemophilia A is caused by a deficiency of factor VIII, but it typically presents with a more severe bleeding phenotype, including hemarthrosis and muscle bleeding, which are not mentioned in this patient's history.
**Option C:** Platelet disorders, such as thrombocytopenia or platelet dysfunction, would not explain the patient's prolonged bleeding time or the reduced ristocetin cofactor assay.

**Clinical Pearl / High-Yield Fact**
Von Willebrand disease is the most common inherited bleeding disorder, and it can present with a wide range of symptoms, from mild bleeding after minor trauma to severe bleeding after surgery or dental procedures. The ristocetin cofactor assay is a critical test for diagnosing VWD, and it can help differentiate it from other bleeding disorders.

**Correct Answer: C. Von Willebrand disease**