**Core Concept**
Venoocclusive disease, also known as sinusoidal obstruction syndrome, is a condition characterized by obstruction of small hepatic veins and sinusoids due to fibrosis and thrombosis. This leads to congestion and necrosis of hepatocytes, resulting in liver dysfunction.

**Why the Correct Answer is Right**
The pathophysiology of venoocclusive disease involves the formation of thrombi within the small hepatic veins and sinusoids, leading to obstruction of blood flow. This is often triggered by toxic substances such as pyrrolizidine alkaloids from certain plants, as well as chemotherapy agents like busulfan. The obstruction of blood flow leads to congestion and ischemia of hepatocytes, resulting in liver dysfunction.

**Why Each Wrong Option is Incorrect**
**Option A:** While Budd-Chiari syndrome is a condition that affects the hepatic veins, it is characterized by thrombosis of the larger hepatic veins, not the small hepatic veins and sinusoids.

**Option B:** Sinusoidal dilatation is a feature of some liver diseases, but it is not a characteristic of venoocclusive disease.

**Option C:** While fibrosis is a feature of venoocclusive disease, it is not the primary cause of the disease.

**Clinical Pearl / High-Yield Fact**
Venoocclusive disease is often seen in patients who have undergone hematopoietic stem cell transplantation. This is because the conditioning regimen used in these patients can include toxic substances that trigger the development of venoocclusive disease.

**Correct Answer: C. Fibrosis**