**Core Concept**
A partial molar pregnancy is a type of gestational trophoblastic disease characterized by an abnormality in the number of chromosomes, whereas a complete molar pregnancy is typically diploid but often shows a paternal origin of the chromosomes. The distinction between these two conditions is crucial for diagnosis and management.

**Why the Correct Answer is Right**
Partial molar pregnancies are typically triploid, meaning they have 69 chromosomes, whereas complete molar pregnancies are usually diploid, with 46 chromosomes. The triploid nature of partial molar pregnancies is due to the presence of two sets of paternal chromosomes and one set of maternal chromosomes. This abnormality in chromosome number leads to the characteristic features of partial molar pregnancies, including an increased risk of complications and a higher likelihood of malignant transformation.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because partial molar pregnancies can show villous formation, but it is often hydropic and disorganized, rather than completely absent. Additionally, the trophoblastic proliferation in partial molar pregnancies is often more disorganized and irregular than in complete molar pregnancies.
**Option B:** While cellular atypia can be seen in both partial and complete molar pregnancies, it is not a distinguishing feature between the two. In fact, cellular atypia is often more pronounced in complete molar pregnancies.
**Option C:** This statement is incorrect because partial molar pregnancies are actually less prone to tumor malignancy than complete molar pregnancies. However, both conditions carry an increased risk of gestational trophoblastic neoplasia compared to normal pregnancies.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that partial molar pregnancies are often associated with an increased risk of complications, including preterm labor, placental abruption, and fetal demise. Additionally, partial molar pregnancies are more likely to be diagnosed incidentally at the time of termination of pregnancy or after a miscarriage.

✓ Correct Answer: D. Partial mole is triploid.