What is the chromosome number of a partial hydatidiform mole?

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

Mnemonics

PHM vs Complete Mole (FETAL) Fetal tissue present = Partial mole (69 XXY, triploidy). Entire paternal = Complete mole (46 XX/XY). Two sperm + one egg = Partial. All paternal duplication = Complete. Lower 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)_