**Core Concept**
Dermoid cysts are a type of mature teratoma that can occur in various parts of the body, including the ovary. They are composed of a mixture of tissues, including hair, skin, and teeth, which are derived from all three embryonic germ layers. The most common complication of dermoid cysts is related to their potential to rupture, leading to severe consequences.

**Why the Correct Answer is Right**
The most common complication of dermoid cyst rupture is **infection**, which can lead to peritonitis and sepsis. This occurs because dermoid cysts can contain a variety of foreign materials, such as hair, bone, and teeth, which can become a source of infection when they rupture. The infection is often polymicrobial, involving a combination of aerobic and anaerobic bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Malignant transformation is a rare complication of dermoid cysts, and it is not the most common complication.
**Option B:** Hemorrhage is a possible complication of dermoid cyst rupture, but it is not the most common complication.
**Option C:** Ovarian torsion is a potential complication of large ovarian cysts, including dermoid cysts, but it is not the most common complication of dermoid cyst rupture.

**Clinical Pearl / High-Yield Fact**
Dermoid cysts are often referred to as "tear-drop" cysts due to their characteristic appearance on ultrasound. They can be difficult to diagnose preoperatively, and rupture is a significant complication that requires prompt surgical intervention.

**Correct Answer: D. Infection. Infection.**