A 6-year-old child presents with a 2-day history of mild fever, headache, and malaise. On examination, the structure marked **A** in the diagram is noted—bright red, hot, edematous bilateral cheeks with sharp demarcation and circumoral pallor. The child is otherwise well. Which of the following is the most likely diagnosis?

Explanation

## Why Erythema infectiosum (Fifth disease) caused by parvovirus B19 is right The bright red, hot, edematous bilateral malar erythema with sharp demarcation and circumoral pallor (marked **A**) is pathognomonic for erythema infectiosum (fifth disease), caused by parvovirus B19. This "slapped-cheek" appearance occurs in the second phase of illness, following a 2–3 day prodrome of mild fever, headache, and myalgia. The rash is typically followed by a reticular or lacy pattern on the trunk and extremities. Notably, the patient is no longer infectious by the time the rash appears, distinguishing it from measles and varicella. (Nelson 21e Ch 273) ## Why each distractor is wrong - **Scarlet fever caused by Group A Streptococcus**: Presents with a fine, sandpaper-like erythematous rash that typically starts on the chest and spreads to the trunk and extremities. The facial rash in scarlet fever is characterized by flushing with circumoral pallor, but lacks the sharp demarcation and intense bilateral malar erythema ("slapped-cheek" appearance) seen in erythema infectiosum. - **Measles caused by measles virus**: While measles does present with fever and rash, the rash begins on the face and spreads downward (cephalocaudal), and is preceded by pathognomonic Koplik spots (white spots on buccal mucosa). The facial erythema is not the characteristic bright red, hot, edematous bilateral cheeks with sharp demarcation. Additionally, measles patients remain infectious during the rash phase, unlike parvovirus B19. - **Rubella caused by rubella virus**: Presents with a fine, pink maculopapular rash that starts on the face and spreads downward, but the facial involvement lacks the intense, hot, edematous bilateral malar erythema with sharp demarcation characteristic of the "slapped-cheek" sign. **High-Yield:** The "slapped-cheek" appearance (bright red, hot, edematous bilateral cheeks with sharp demarcation + circumoral pallor) is pathognomonic for erythema infectiosum caused by parvovirus B19, and the patient is non-infectious by the time the rash appears. [cite: Nelson 21e Ch 273]