What is the chromosome number of a partial hydatidiform mole?

Explanation

## Correct Answer: B. 69 XXY A partial hydatidiform mole (PHM) arises from **triploidy** — specifically, the karyotype **69 XXY** (or 69 XXX or 69 XYY, but 69 XXY is most common). This occurs when a normal haploid ovum (23 X) is fertilized by **two sperm** (each contributing 23 chromosomes), resulting in 69 total chromosomes with an extra paternal contribution. Genetically, PHM is diandric triploidy (two paternal sets, one maternal set). Clinically, PHM presents with **both fetal tissue and abnormal trophoblastic tissue** — unlike complete mole (46 XX or 46 XY, all paternal) which has no fetus. The fetus in PHM is usually severely malformed and dies in utero; the pregnancy may progress to second trimester before diagnosis. On ultrasound, PHM shows a **hydropic fetus with placental abnormalities** (cystic spaces smaller than complete mole). Histologically, there is **focal trophoblastic hyperplasia** with some normal villi interspersed. The risk of persistent gestational trophoblastic disease (GTD) is lower (~1–5%) than complete mole (~15–20%), but still requires β-hCG monitoring post-evacuation per Indian guidelines (FIGO/ICMR protocols). The 69 XXY karyotype is the discriminating feature that separates PHM from complete mole and normal pregnancy. ## Why the other options are wrong **A. 46 XX** — This is the normal female karyotype and represents a **euploid diploid state**. While a complete hydatidiform mole can be 46 XX (all paternal origin, no fetal tissue), it is NOT the karyotype of a partial mole. A 46 XX complete mole arises from duplication of a single sperm's chromosomes, not from triploidy. This option confuses complete mole with partial mole — a classic NBE trap. **C. 69 XY** — This represents **triploidy with XY sex chromosomes** (69 XYY or 69 XYX), which can occur but is **not the most common karyotype** in partial moles. While 69 XY triploidy can theoretically produce a PHM, the question asks for THE chromosome number of a partial mole, implying the most frequent presentation. 69 XXY is more commonly reported in Indian and international literature. This option tests whether students confuse the sex chromosome composition. **D. 45 XY** — This is **monosomy X (Turner syndrome karyotype)** — a **hypoploidy**, not triploidy. This karyotype is incompatible with hydatidiform mole formation and is associated with Turner syndrome in live births (though usually lethal in utero). This option is a distractor testing whether students understand the difference between triploidy (extra set) and monosomy (missing chromosome). ## High-Yield Facts - **Partial hydatidiform mole = 69 XXY (triploidy)** — two paternal, one maternal chromosome set (diandric triploidy). - **Complete mole = 46 XX or 46 XY** — all paternal origin, no fetal tissue, higher GTD risk (~15–20%). - **Partial mole has fetal tissue + abnormal placenta** — hydropic fetus visible on ultrasound, focal trophoblastic hyperplasia on histology. - **Persistent GTD risk: Complete mole ~15–20%, Partial mole ~1–5%** — both require post-evacuation β-hCG monitoring per ICMR guidelines. - **Triploidy (69 XXX, 69 XXY, 69 XYY) incompatible with life** — all triploid pregnancies result in fetal loss; PHM is the only viable presentation. ## Mnemonics **PHM vs Complete Mole (FETAL)** **F**etal tissue present = Partial mole (69 XXY, triploidy). **E**ntire paternal = Complete mole (46 XX/XY). **T**wo sperm + one egg = Partial. **A**ll paternal duplication = Complete. **L**ower GTD risk = Partial (~5%), Higher = Complete (~20%). **Triploidy Karyotypes (3 Sets)** **69 XXY** (most common PHM), **69 XXX**, **69 XYY** — all are triploidy. Remember: 69 = 3 sets of 23. Partial mole = 2 paternal + 1 maternal (diandric). Complete mole = 0 maternal + 2 paternal (androgenetic). ## NBE Trap NBE pairs "hydatidiform mole" with "46 XX" to trap students who conflate complete mole (46 XX, all paternal) with partial mole (69 XXY, triploidy). The key discriminator is **fetal tissue presence** — if fetus is present, think triploidy (69); if no fetus, think complete mole (46). ## Clinical Pearl In Indian obstetric practice, a second-trimester ultrasound showing a **hydropic fetus with cystic placental spaces** should immediately raise suspicion for partial mole — order karyotyping and β-hCG levels. Post-evacuation, these patients require 6–12 months of β-hCG monitoring (weekly × 3, then monthly) to detect persistent GTD, as per ICMR guidelines, even though their GTD risk is lower than complete mole. _Reference: DC Dutta's Textbook of Obstetrics (3rd ed), Ch. 24 (Gestational Trophoblastic Disease); Harrison's Principles of Internal Medicine, Ch. 105 (Gestational Trophoblastic Neoplasia)_