Dermatology MCQ - Viral Infections - Erythema infectiosum

A 7-year-old child presents with bright red, slapped cheeks and a lacy, reticular rash on the extremities. The child is afebrile and otherwise well. The mother reports that several classmates had similar symptoms. Which of the following is the most likely causative agent and the associated risk in pregnant women?

A) Human herpesvirus 6; risk of fetal encephalitis
B) Parvovirus B19; risk of fetal hydrops
C) Rubella virus; risk of congenital rubella syndrome
D) Measles virus; risk of fetal miscarriage
E) Enterovirus; risk of fetal myocarditis

Correct Answer: B) Parvovirus B19; risk of fetal hydrops

Explanation

This presentation is classic for erythema infectiosum (fifth disease), caused by parvovirus B19.

Key Clinical Features of Erythema Infectiosum:

• "Slapped Cheek" Rash: Bright red erythema on the cheeks, sparing the nasolabial folds and circumoral area.

• Lacy Reticular Rash: A pink, net-like (reticular) rash on the trunk and extremities, which may wax and wane with heat, exercise, or stress.

• Constitutional Symptoms: Typically mild or absent; children are usually afebrile and feel well.

• Epidemiology: Common in school-aged children; outbreaks occur in schools or daycare centers.

Virologic Features:

• Causative Agent: Parvovirus B19, a single-stranded DNA virus.

• Transmission: Respiratory droplets; incubation period is 4-14 days.

Risk in Pregnant Women:

• Fetal Hydrops: Parvovirus B19 can cross the placenta and infect fetal erythroid precursor cells, causing severe anemia, high-output cardiac failure, and non-immune hydrops fetalis (fluid accumulation in fetal tissues).

• Timing: Highest risk during the first half of pregnancy.

• Management: Serial ultrasounds and Doppler studies to monitor for hydrops; intrauterine transfusion may be needed if severe anemia develops.

Why Not the Other Options?

• (A) Human herpesvirus 6: Causes roseola infantum (high fever followed by diffuse rash), not slapped cheek rash. No specific fetal risks.

• (C) Rubella virus: Causes rubella (pink rash, lymphadenopathy) and congenital rubella syndrome (cardiac defects, cataracts, deafness), not erythema infectiosum.

• (D) Measles virus: Causes measles (Koplik spots, cephalocaudal rash), not reticular rash. Associated with fetal miscarriage but not hydrops.

• (E) Enterovirus: Causes hand, foot, and mouth disease (vesicles on hands/feet/mouth) or myocarditis, not slapped cheek rash.

Complications:

• In children: Usually self-limited; may cause transient arthritis/arthralgia.

• In immunocompromised hosts: Can cause pure red cell aplasia.

• In adults: May cause arthralgia (especially women).

Management:

• Supportive care; no specific antiviral treatment.

• Pregnant women exposed to parvovirus B19 should have serologic testing (IgM/IgG) and ultrasound monitoring.

Prognosis:
Excellent in healthy children; resolves in 1-3 weeks.