## **Core Concept**
Choriocarcinoma and invasive mole (chorio-adenoma destruens) are both gestational trophoblastic diseases, but they have distinct histological and clinical features. The key to differentiating between them lies in their cellular composition and behavior. Choriocarcinoma is a malignant tumor that consists of cytotrophoblast and syncytiotrophoblast without chorionic villi, whereas an invasive mole invades the myometrium but retains chorionic villi.

## **Why the Correct Answer is Right**
The correct answer, **D. Absence of chorionic villi**, highlights a critical histopathological distinction. Choriocarcinoma is characterized by the presence of both cytotrophoblast and syncytiotrophoblast but lacks chorionic villi. This absence of villi is a hallmark that distinguishes choriocarcinoma from an invasive mole, which does contain chorionic villi despite its invasive nature into the myometrium.

## **Why Each Wrong Option is Incorrect**
- **Option A: Presence of cytotrophoblast** - While cytotrophoblasts are a component of choriocarcinoma, they are also found in normal placental development and other gestational trophoblastic diseases, making this not specific enough for differentiation.
- **Option B: Presence of syncytiotrophoblast** - Like cytotrophoblasts, syncytiotrophoblasts are present in choriocarcinoma but are also a normal component of placental tissue, including in conditions like invasive mole.
- **Option C: Human chorionic gonadotropin (hCG) levels** - Although hCG levels can be elevated in both conditions, the degree of elevation and the response to treatment can vary. However, this is not a definitive way to differentiate between choriocarcinoma and invasive mole histologically.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that choriocarcinoma often presents with a higher risk of metastasis and may require more aggressive chemotherapy compared to invasive mole. The absence of chorionic villi in choriocarcinoma is not just a histological feature but also correlates with its potential for more aggressive behavior.

## **Correct Answer: D. Absence of chorionic villi**