**Core Concept**
Red degeneration of fibroid is a rare but serious condition that occurs during pregnancy, typically in the second or third trimester. It is characterized by the sudden onset of abdominal pain, low-grade fever, and mild leukocytosis, due to the degeneration of a fibroid uterus.

**Why the Correct Answer is Right**
Red degeneration of fibroid is thought to occur when the fibroid outgrows its blood supply, leading to ischemia and subsequent necrosis. This process triggers an inflammatory response, causing the characteristic symptoms of abdominal pain, low-grade fever, and mild leukocytosis. The condition is often associated with a sudden increase in fibroid size, which can occur due to hormonal changes during pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Preterm labor is a condition characterized by the onset of labor before 37 weeks of gestation, often accompanied by cervical dilation and contractions. While preterm labor can cause abdominal pain, it is typically associated with a more rapid progression of labor and often presents with cervical changes.

**Option B:** Torsion of fibroid is a rare condition where a fibroid becomes twisted, cutting off its blood supply. This can cause severe abdominal pain, but it is typically associated with a more sudden and severe onset of pain, often accompanied by nausea and vomiting.

**Option D:** Infection in fibroid is a possible complication of fibroid degeneration, but it is not the most likely diagnosis in this scenario. Infection would typically be accompanied by more severe symptoms, such as high fever, chills, and a more pronounced leukocytosis.

**Clinical Pearl / High-Yield Fact**
Red degeneration of fibroid is often associated with a sudden increase in fibroid size, which can be mistaken for preterm labor or other obstetric complications. It is essential to consider this diagnosis in pregnant women with fibroids who present with acute abdominal pain and mild leukocytosis.

**✓ Correct Answer: C. Red degeneration of fibroid**