**Core Concept**
Red degeneration of a fibroid is a rare but serious complication of uterine leiomyomas (fibroids). It is characterized by sudden, severe abdominal pain due to ischemic necrosis of the fibroid tissue. This process is often associated with rapid growth of the fibroid, which outstrips its blood supply.

**Why the Correct Answer is Right**
The correct answer is **B. Pregnancy**. Red degeneration of a fibroid is most commonly seen in pregnant women, particularly during the second trimester. The rapid growth of the fibroid, combined with increased uterine distension and changes in uteroplacental blood flow, can lead to ischemia and necrosis of the fibroid tissue. This condition is thought to be caused by the sudden increase in uterine blood flow and the resulting vascular steal phenomenon, which reduces blood flow to the fibroid.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because red degeneration of a fibroid can occur in non-pregnant women, but it is much less common than in pregnant women. While hormonal changes and rapid growth can still contribute to ischemia, the underlying pathophysiology is different from that in pregnant women.
**Option C:** This option is incorrect because red degeneration of a fibroid is not typically associated with the use of oral contraceptives. In fact, hormonal contraceptives may even reduce the growth of fibroids and alleviate symptoms.

**Clinical Pearl / High-Yield Fact**
To remember the correct answer, think of the "Pregnancy-Fibroid" connection: "P" for Pregnancy, which increases the risk of red degeneration, and "F" for Fibroid, which is the underlying cause of the condition.

**Correct Answer:** B. Pregnancy