**Core Concept**
Hydatidiform mole, also known as molar pregnancy, is a rare abnormality of early pregnancy characterized by the proliferation of trophoblast cells, which form a mass of grape-like vesicles. This condition often presents with vaginal bleeding, uterine enlargement, and elevated human chorionic gonadotropin (hCG) levels.

**Why the Correct Answer is Right**
The "snowstorm pattern" observed on intravaginal ultrasound is a hallmark of hydatidiform mole, resulting from the presence of multiple small, hyperechoic vesicles within the uterine cavity. This pattern, combined with the absence of a fetal heartbeat and markedly elevated hCG levels, supports the diagnosis. Additionally, the uterine enlargement disproportionate to the gestational age and vaginal bleeding are common presenting symptoms of hydatidiform mole.

**Why Each Wrong Option is Incorrect**
**Option A:** Preeclampsia is characterized by hypertension and proteinuria after 20 weeks of gestation, which does not match the presentation in this case.

**Option B:** Eclampsia is the onset of seizures in a woman with preeclampsia, which is not indicated in this scenario.

**Option C:** Ectopic pregnancy would typically present with abdominal pain and a negative beta-hCG level, which is not consistent with this case.

**Clinical Pearl / High-Yield Fact**
Hydatidiform mole is a type of gestational trophoblastic disease, which can be divided into two subtypes: complete mole (characterized by a 46,XX karyotype with no fetal tissue) and partial mole (characterized by a triploid karyotype with fetal tissue present). Recognizing these differences is crucial for accurate diagnosis and management.

**✓ Correct Answer: D. Hydatidiform mole.**