**Core Concept**
Red degeneration of fibroid is a condition characterized by sudden onset of abdominal pain, fever, and leucocytosis in a pregnant woman with a fibroid uterus. This condition is often associated with fibroids undergoing rapid growth and degeneration.

**Why the Correct Answer is Right**
Red degeneration of fibroid occurs due to the sudden cessation of blood supply to the rapidly growing fibroid, leading to ischemic necrosis. This condition is often seen in the second trimester of pregnancy. The pain is usually severe and can radiate to the back, accompanied by fever and leucocytosis. The exact pathophysiology is not well understood, but it is thought to be related to the increased demand for oxygen and nutrients by the rapidly growing fibroid.

**Why Each Wrong Option is Incorrect**
**Option A:** Preterm labour is characterized by the onset of labor before 37 weeks of gestation, which is not directly related to fibroid degeneration. While fibroids can contribute to preterm labour by causing uterine distension, this is not the primary diagnosis in this scenario.

**Option B:** Torsion of fibroid is a rare condition that can occur in pregnant women, but it is not as common as red degeneration of fibroid. The presentation is usually acute, with severe pain and possibly a palpable mass, but it does not typically involve fever and leucocytosis.

**Option D:** Infection in fibroid is a possible complication, but it is not the most likely diagnosis in this scenario. The presence of fever and leucocytosis suggests an inflammatory process, but red degeneration of fibroid is a more common and well-recognized cause of these symptoms in pregnant women with fibroids.

**Clinical Pearl / High-Yield Fact**
Red degeneration of fibroid is a classic exam trap, as it can be easily confused with other conditions such as preterm labour or torsion of fibroid. However, the presence of fever and leucocytosis, along with severe abdominal pain, should raise suspicion for red degeneration of fibroid in a pregnant woman with a fibroid uterus.

✓ Correct Answer: C. Red degeneration of fibroid