A 7-year-old girl presents with a 3-day history of fever, malaise, and a bright red facial rash that appears as if she has been slapped on the cheeks. The rash spares the nasolabial folds and oral mucosa. Over the next 2 days, a lacy, reticular rash appears on her trunk and extremities that blanches with pressure. She has mild joint pain in her knees and wrists. Her mother reports that several children in her school developed similar illness 2 weeks ago. Full blood count shows mild anemia (Hb 10.2 g/dL) with reticulocytosis. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Erythema Infectiosum (Fifth Disease) ### Key Clinical Features **Key Point:** Erythema infectiosum is caused by parvovirus B19 and is characterized by the pathognomonic "slapped cheek" appearance followed by a lacy, reticular rash on the trunk and extremities. ### Characteristic Rash Pattern The rash evolves in three phases: 1. **Phase 1 (Days 1–3):** Bright erythema of cheeks ("slapped cheek" appearance), sparing nasolabial folds and oral mucosa 2. **Phase 2 (Days 3–7):** Lacy, reticular rash on trunk and extremities; may blanch with pressure 3. **Phase 3 (Days 7–14):** Rash may reappear with fever, stress, or sun exposure ### Associated Findings | Feature | Presence in Erythema Infectiosum | | --- | --- | | Fever | Mild, often absent by rash onset | | Arthralgia/arthritis | Common in children (knees, wrists, ankles) | | Lymphadenopathy | Minimal | | Oral involvement | Absent (rash spares oral mucosa) | | Reticulocytosis | Present (due to transient RBC aplasia) | **High-Yield:** The combination of "slapped cheek" rash + lacy reticular rash + reticulocytosis is pathognomonic for parvovirus B19. ### Pathophysiology Parvovirus B19 is a small, non-enveloped DNA virus that: - Infects erythroid progenitor cells in bone marrow - Causes transient arrest of RBC production (reticulocytopenia followed by reticulocytosis) - In immunocompetent children, this is usually asymptomatic; in those with hemolytic anemias (sickle cell, hereditary spherocytosis), it can cause aplastic crisis **Clinical Pearl:** In children with chronic hemolytic anemia, parvovirus B19 can precipitate a life-threatening aplastic crisis with severe anemia and reticulocytopenia. ### Epidemiology - Most common in children aged 5–15 years - Peak incidence in spring - Highly contagious; spreads via respiratory droplets - Schoolchildren are a common source of transmission ### Complications in Special Populations - **Pregnant women (especially first/second trimester):** Risk of hydrops fetalis and fetal loss (10–15%) - **Immunocompromised patients:** Chronic anemia and persistent viremia - **Patients with hemolytic anemias:** Aplastic crisis **Mnemonic:** **FIFTH DISEASE** = **F**ace slapped, **I**nfectious (parvovirus B19), **F**ever mild, **T**hen lacy rash, **H**emolytic risk in sickle cell ### Management 1. **Supportive care:** Antipyretics, fluids, rest 2. **Monitoring:** Serial hemoglobin in high-risk patients (hemolytic anemias) 3. **Isolation:** Not required after rash appears (non-infectious by then) 4. **Pregnancy:** Fetal monitoring if maternal infection in first/second trimester [cite:Park 26e Ch 16]