A 4-year-old boy from rural Maharashtra presents with a 3-day history of fever, cough, and coryza. On examination, he has a temperature of 39.5°C, conjunctival injection, and oral ulcers with white spots on the buccal mucosa opposite the molars. His mother reports that he has not been vaccinated. On day 4 of illness, he develops a maculopapular rash that begins on the forehead and hairline, spreading downward to the trunk and limbs. The rash blanches on pressure. What is the most likely diagnosis?

Explanation

## Diagnosis: Measles (Rubeola) ### Clinical Presentation Breakdown **High-Yield:** The combination of **Koplik spots** (white spots on buccal mucosa opposite molars) with the **3 Cs** (cough, coryza, conjunctivitis) in the prodromal phase is pathognomonic for measles. **Key Point:** Measles follows a predictable temporal sequence: 1. **Prodromal phase (3–4 days):** Fever, cough, coryza, conjunctivitis, and Koplik spots 2. **Exanthem phase (3–7 days):** Maculopapular rash appearing on the 4th–5th day of illness ### Rash Characteristics | Feature | Measles | Rubella | Varicella | |---------|---------|---------|----------| | **Onset location** | Hairline/forehead → spreads downward | Face → trunk | Crops on trunk first | | **Morphology** | Maculopapular, blanching | Fine maculopapular | Vesicular ("dew drops on rose petal") | | **Confluence** | Yes, especially on face | No | Non-confluent | | **Timing** | Day 4–5 of illness | Day 1–2 of illness | Simultaneous with fever onset | ### Why This Is Measles **Clinical Pearl:** The **cephalocaudal (downward) spread** of the rash over 3–4 days is characteristic of measles and helps distinguish it from rubella (which appears all at once) and varicella (which appears in crops). **Key Point:** Koplik spots are **pathognomonic** — they appear 2–3 days before the rash and fade as the rash emerges. No other common viral exanthem produces them. ### Epidemiological Context **High-Yield:** Unvaccinated children in India remain at high risk for measles, which is still endemic in many rural areas. The absence of vaccination history is a critical risk factor. [cite:Park 26e Ch 7]