**Core Concept**
Prolonged latent phase is a significant concern in obstetrics, particularly in nulliparous women, as it increases the risk of prolonged labor, fetal distress, and cesarean delivery. The latent phase is the initial period of labor, characterized by cervical dilation, but without the active, intense contractions that follow. The World Health Organization (WHO) defines prolonged latent phase as a duration exceeding 20 hours in nulliparous women.

**Why the Correct Answer is Right**
The correct answer is based on the WHO definition for prolonged latent phase in nulliparous women. This definition is crucial for obstetricians to identify cases that may require intervention to prevent complications. The prolonged latent phase is often associated with a longer duration of labor, increased risk of fetal distress, and higher rates of cesarean delivery.

**Why Each Wrong Option is Incorrect**
**Option A:** 12 hours - This duration is not considered prolonged latent phase, as it falls within the normal range for nulliparous women.
**Option B:** 24 hours - While this duration is closer to the threshold, it is still not the official cutoff for prolonged latent phase in nulliparous women.
**Option C:** 36 hours - This option is incorrect because it exceeds the normal duration of labor, but it is not the specific cutoff for prolonged latent phase in nulliparous women.

**Clinical Pearl / High-Yield Fact**
The WHO definition of prolonged latent phase is a critical guideline for obstetricians to identify cases that may require intervention. Remembering this definition can help you differentiate between normal and prolonged labor, enabling you to provide timely and appropriate care.

**Correct Answer: D. 20 hours**