**Core Concept**
ADAMTS-13, also known as von Willebrand factor-cleaving protease, is a metalloprotease that plays a crucial role in regulating the activity of von Willebrand factor (VWF) by cleaving it into smaller fragments. This process is essential in preventing excessive platelet aggregation and thrombosis.

**Why the Correct Answer is Right**
ADAMTS-13 is primarily produced by the endothelial cells lining the blood vessels. These cells produce ADAMTS-13 as a zymogen, which is then activated to its metalloprotease form. The enzyme then cleaves VWF at a specific site, reducing its ability to bind platelets and thereby preventing excessive platelet aggregation. This process is critical in maintaining normal hemostasis.

**Why Each Wrong Option is Incorrect**
**Option A:** While von Willebrand factor is indeed a substrate for ADAMTS-13, it is not the primary site of ADAMTS-13 production. The endothelial cells are responsible for producing this enzyme.
**Option B:** Platelets do not produce ADAMTS-13. Instead, they are affected by the actions of this enzyme, which cleaves VWF and reduces its ability to bind platelets.
**Option C:** The liver does produce various metalloproteases, but it is not the primary source of ADAMTS-13.

**Clinical Pearl / High-Yield Fact**
ADAMTS-13 deficiency is associated with thrombotic thrombocytopenic purpura (TTP), a rare bleeding disorder characterized by the formation of blood clots in small blood vessels throughout the body. TTP is often caused by ADAMTS-13 autoantibodies that inhibit the enzyme's activity.

**Correct Answer:** C. Endothelial cells.