A 28-year-old primigravida presents with painless vaginal bleeding and severe nausea at 12 weeks gestation. Transvaginal ultrasound reveals a uterus enlarged for dates with a characteristic "snowstorm" appearance and no identifiable fetal pole. The structure marked **A** in the diagram shows the hydropic, grape-like chorionic villi typical of this condition. Bilateral ovarian cysts are also noted. Which of the following statements best characterizes the chromosomal composition and malignant potential of this lesion?

Explanation

## Why Option 1 is correct The clinical presentation—painless vaginal bleeding, uterus large for dates, "snowstorm" ultrasound appearance, no fetal pole, and bilateral ovarian cysts from markedly elevated β-hCG—is pathognomonic for a **complete hydatidiform mole**. The structure marked **A** (hydropic, grape-like chorionic villi) is the hallmark histological finding. In a complete mole, the empty ovum is fertilized by a single sperm that duplicates, resulting in **46,XX paternal chromosomes in ~90% of cases** (the remaining 10% are 46,XY paternal). All chromosomes are of paternal origin; there is no embryonic tissue. The **malignant potential is ~15–20%**, with progression to choriocarcinoma being the most serious sequela. This high risk necessitates strict follow-up with serial β-hCG monitoring and contraception during surveillance (Williams Obstetrics 26e). ## Why each distractor is wrong - **Option 2 (Partial mole, 69,XXY, ~5% risk)**: Partial moles result from diandry (normal egg fertilized by two sperms or a diploid sperm), producing **triploidy (69,XXY or 69,XXX)**. Fetal tissue is often present but anomalous. The malignant potential is only ~5%. The clinical picture here—no fetal pole, massive β-hCG elevation, bilateral theca-lutein cysts—is inconsistent with a partial mole. - **Option 3 (Complete mole, 46,XY, ~25% risk)**: While 46,XY complete moles do occur (~10% of cases), the malignant potential of complete moles is ~15–20%, not 25%. The question stem points to the classic 46,XX form (90% of complete moles). - **Option 4 (Partial mole, 46,XX maternal, ~10% risk)**: Partial moles are triploidy, not diploid. Maternal chromosomes alone cannot produce a viable pregnancy. This option conflates complete and partial mole genetics and misrepresents the risk stratification. **High-Yield:** Complete hydatidiform mole = 46,XX paternal (90%) or 46,XY paternal (10%), NO embryo, "snowstorm" on USG, ~15–20% choriocarcinoma risk; partial mole = 69,XXY/XXX triploidy, fetal tissue present but anomalous, ~5% risk. [cite: Williams Obstetrics 26e — Gestational Trophoblastic Disease chapter]