**Core Concept**
Von Willebrand's disease (VWD) is a congenital bleeding disorder characterized by a deficiency or dysfunction of von Willebrand factor (VWF), a crucial protein involved in primary hemostasis. VWF plays a key role in platelet adhesion to injured blood vessels and serves as a carrier protein for factor VIII, protecting it from premature degradation. The deficiency or dysfunction of VWF leads to impaired platelet aggregation and abnormal bleeding manifestations.

**Why the Correct Answer is Right**
In patients with VWD, the coagulation profile typically shows a prolonged bleeding time and a decreased or absent ristocetin cofactor activity. The partial thromboplastin time (PTT) is often prolonged, primarily due to the decreased activity of factor VIII, which is carried by VWF. The prothrombin time (PT) is usually normal, as the extrinsic coagulation pathway is intact. The coagulation profile may also show a decreased VWF antigen level or a decreased factor VIII activity level.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it does not accurately represent the coagulation profile in VWD. While PTT may be prolonged, PT is typically normal.
* **Option B:** This option is incorrect as it does not accurately reflect the coagulation profile in VWD. A prolonged bleeding time is a characteristic feature of VWD, but PT is usually normal.
* **Option D:** This option is incorrect as it does not accurately represent the coagulation profile in VWD. While factor VIII activity may be decreased, PT is typically normal.

**Clinical Pearl / High-Yield Fact**
In patients with VWD, the severity of bleeding manifestations can vary widely, and some individuals may remain asymptomatic throughout their lives. A careful family history and physical examination are essential in diagnosing VWD, as laboratory tests may not always be abnormal.

**Correct Answer:** C. Prolonged PTT, normal PT, decreased VWF antigen level or decreased factor VIII activity level.