## **Core Concept**
The condition described, where a patient has two uteri, is suggestive of **uterus didelphys**, a type of Müllerian duct anomaly. This anomaly results from an issue during embryological development, specifically involving the Müllerian ducts, which are precursor structures to the female reproductive tract.

## **Why the Correct Answer is Right**
The correct answer, **D. Failure of the Müllerian ducts to fuse**, explains the presence of two separate uteri, a condition known as uterus didelphys. During normal embryological development, the Müllerian ducts fuse to form the uterus, fallopian tubes, cervix, and the upper part of the vagina. If the Müllerian ducts fail to fuse, two separate uterine structures develop. This condition can be associated with a double cervix and sometimes a double vagina. The failure of fusion occurs around 6-10 weeks of gestation.

## **Why Each Wrong Option is Incorrect**
- **Option A: Failure of one Müllerian duct to develop** would result in **uterus unicornis** or a unicornuate uterus, where only one side of the uterus develops.
- **Option B: Failure of the Müllerian ducts to regress** is related to **persistent Müllerian duct syndrome**, a condition seen in males where the Müllerian ducts fail to regress due to a lack of Müllerian inhibiting substance, leading to the development of female reproductive structures in an otherwise male individual.
- **Option C: Abnormal implantation of the embryo** does not relate to the embryological development of the uterus itself but rather to the implantation process of the fertilized egg.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that patients with Müllerian duct anomalies, including uterus didelphys, may have a higher risk of complications during pregnancy, such as preterm labor, breech presentation, and difficulties during delivery. Additionally, these anomalies can be associated with renal and vaginal anomalies, emphasizing the importance of thorough evaluation.

## **Correct Answer:** D. Failure of the Müllerian ducts to fuse.