**Core Concept**
Uterine leiomyomas, also known as uterine fibroids, are benign tumors of the smooth muscle layer of the uterus. They are common in women of reproductive age and are influenced by hormonal factors, particularly estrogen.

**Why the Correct Answer is Right**
The correct answer is Malignant transformation. Uterine leiomyomas are rare to undergo malignant transformation, but when they do, they are called leiomyosarcomas. This transformation is often associated with a higher risk of metastasis and poor prognosis. The exact mechanism of malignant transformation is not fully understood, but it is thought to involve genetic mutations, such as alterations in the TP53 gene.

**Why Each Wrong Option is Incorrect**
**Option A:** Degeneration is a common complication of uterine leiomyomas, leading to symptoms such as pain and heavy menstrual bleeding. Degeneration can occur due to various factors, including hormonal changes, trauma, or infection.

**Option B:** Red degeneration is a specific type of degeneration that occurs in uterine leiomyomas during pregnancy. It is characterized by pain, fever, and uterine tenderness, and is thought to be caused by hormonal changes and ischemia.

**Option C:** Sarcomatous transformation is a rare but serious complication of uterine leiomyomas. It involves the transformation of the benign tumor into a malignant leiomyosarcoma, which can lead to metastasis and poor prognosis.

**Clinical Pearl / High-Yield Fact**
Uterine leiomyomas are often asymptomatic, but when they cause symptoms, it is usually due to degeneration or red degeneration. It is essential to differentiate these conditions from other causes of pelvic pain or abnormal uterine bleeding.

**Correct Answer: C. Malignant transformation.**