**Question:** Least common type of twins

Core Concept: Types of twins and their rarity

Twins are categorized into monozygotic (MZ) and dizygotic (DZ) twins based on the number of fertilized eggs and subsequent embryo development. Monozygotic twins arise from one fertilized egg that divides into two embryos, while dizygotic twins are formed from two separate fertilized eggs.

**Why the Correct Answer is Right:**

The least common type of twins is monozygotic (MZ) twins. This occurs when a single fertilized egg divides into two embryos, resulting in identical twins with the same genetic makeup. In comparison, dizygotic (DZ) twins are more common, resulting from two separate fertilized eggs.

**Why Each Wrong Option is Incorrect:**

A. Dizygotic (DZ) twins are not the least common type, as they result from two separate fertilized eggs.
B. Dizygotic (DZ) twins are not the least common type, as they result from two separate fertilized eggs.
C. Dizygotic (DZ) twins are not the least common type, as they result from two separate fertilized eggs.
D. Dizygotic (DZ) twins are not the least common type, as they result from two separate fertilized eggs.

**Clinical Pearl / High-Yield Fact:**

Understanding the distinction between monozygotic (MZ) and dizygotic (DZ) twins is crucial for clinical practice, as the type of twin formation can impact genetic relatedness and clinical management. Monozygotic twins are more likely to share a common placenta, while dizygotic twins may have separate placentas and require individualized care.

**Correct Answer:** Monozygotic (MZ) twins