Correct Answer: A. Parvovirus B19
Parvovirus B19 (erythrovirus) is the classic viral cause of hydrops fetalis that mimics hemolytic disease of the newborn (erythroblastosis fetalis). The virus directly infects and lyses erythroid progenitor cells in the fetal bone marrow, causing severe anemia. Unlike Rh incompatibility or ABO hemolysis (true erythroblastosis), parvovirus-induced hydrops results from anemia-induced high-output cardiac failure rather than immune hemolysis. The infected fetus develops profound anemia (Hb can drop to 2–3 g/dL), leading to compensatory erythropoiesis, hepatosplenomegaly, and fluid accumulation (ascites, pleural effusion, pericardial effusion). Maternal infection in the first and second trimester carries highest risk (10–15% fetal loss). The diagnosis is confirmed by IgM serology or PCR from amniotic fluid. Indian obstetric guidelines emphasize routine serological screening in high-risk pregnancies. The key discriminator is that parvovirus causes non-immune hydrops with preserved platelet count and normal bilirubin (no hemolysis), whereas true erythroblastosis shows hemolytic markers.
Why the other options are wrong
B. HSV — HSV causes disseminated neonatal infection with vesicular rash, hepatitis, and CNS involvement, but does NOT cause hydrops fetalis or mimic erythroblastosis. HSV transmission is typically peripartum (ascending infection or vaginal delivery), not transplacental in early pregnancy. The clinical presentation is acute neonatal sepsis, not chronic anemia-induced hydrops. C. Picornavirus — Picornaviruses (enterovirus, coxsackievirus) cause myocarditis, meningitis, and hand-foot-mouth disease in neonates, but do not selectively target erythroid progenitors or cause non-immune hydrops. They do not mimic the anemia-driven pathophysiology of erythroblastosis fetalis. D. Epstein-Barr virus — EBV causes infectious mononucleosis with lymphadenopathy and splenomegaly, but does not cause congenital infection or fetal hydrops. Congenital EBV is rare and presents with hepatosplenomegaly and jaundice from hepatitis, not from bone marrow suppression mimicking hemolytic disease.
High-Yield Facts
- Parvovirus B19 causes non-immune hydrops fetalis by direct lysis of erythroid progenitors, NOT immune hemolysis.
- Fetal infection risk is highest in first and second trimester (10–15% fetal loss); third trimester carries lower risk.
- Diagnosis: IgM serology or PCR from amniotic fluid; fetal anemia confirmed by cordocentesis (Hb <7 g/dL warrants intrauterine transfusion).
- Parvovirus-induced hydrops shows normal platelet count and bilirubin (no hemolysis), distinguishing it from Rh/ABO incompatibility.
- Intrauterine transfusion is the definitive fetal therapy; maternal IVIG may reduce viral load but does not reverse established anemia.
Mnemonics
PARVOVIRUS = Progenitor destruction Parvovirus B19 destroys erythroid progenitors (not mature RBCs), causing severe anemia → hydrops. Remember: it mimics erythroblastosis because both cause fetal anemia and hydrops, but parvovirus is non-immune (no hemolysis, normal bilirubin). HFM Triad for Parvovirus Hydrops Hydrops (ascites, effusions), Fetal anemia (Hb <7), Maternal IgM positive. Use this to confirm diagnosis and decide on cordocentesis/transfusion.
NBE Trap
NBE pairs "erythroblastosis fetalis" with hemolytic disease to lure students into choosing HSV or EBV (which cause neonatal sepsis/hepatitis). The trap: parvovirus mimics erythroblastosis clinically (hydrops, anemia) but is pathologically non-immune, not a true hemolytic disease.
Clinical Pearl
In Indian obstetric practice, a pregnant woman presenting with polyhydramnios and fetal hydrops on ultrasound, with negative Rh/ABO serology, should raise suspicion for parvovirus B19. Cordocentesis for fetal hemoglobin and parvovirus PCR guides urgent intrauterine transfusion, which can be life-saving in tertiary centers.
_Reference: Jawetz Melnick & Adelberg's Medical Microbiology Ch. 37 (Parvoviruses); Harrison Principles of Internal Medicine Ch. 219 (Parvovirus B19)_