A 4-year-old boy from rural Maharashtra presents with a 3-day history of high fever (39.5°C), cough, coryza, and conjunctivitis. On examination, you note small white spots with red halos on the buccal mucosa opposite the molars. The child has a blanching maculopapular rash that started on the forehead and hairline 2 days ago and is now spreading downward to the trunk and limbs. He is unvaccinated. Which of the following is the most likely diagnosis?

## Clinical Diagnosis: Measles ### Key Clinical Features Present **Key Point:** The pathognomonic finding here is **Koplik spots** — small white spots with red halos on the buccal mucosa opposite the molars, appearing 2–3 days before the rash. **High-Yield:** Koplik spots are virtually diagnostic of measles and appear during the **prodromal phase** (fever, cough, coryza, conjunctivitis — the "3 Cs"). ### Prodromal Phase (First 3–4 days) | Feature | Details | |---------|----------| | **Fever** | High, often 39–40°C | | **Respiratory symptoms** | Cough, coryza, pharyngitis | | **Ocular signs** | Conjunctivitis (photophobia common) | | **Pathognomonic finding** | **Koplik spots** on buccal mucosa | | **Timing** | Appear 2–3 days before rash | ### Exanthem Phase (Day 3–4 onward) - **Maculopapular rash**, blanching, non-pruritic - **Cephalocaudal spread**: starts on forehead/hairline → spreads downward over 3–4 days - **Peak intensity** on day 3–4 of rash - Rash fades in the same order (head first) **Clinical Pearl:** The **cephalocaudal progression** combined with **Koplik spots** in the prodrome is virtually 100% specific for measles. ### Why This Is Measles (Not Other Exanthems) ```mermaid flowchart TD A[Fever + Cough + Coryza + Conjunctivitis]:::outcome --> B{Koplik spots present?}:::decision B -->|Yes| C[Measles]:::action B -->|No| D{Other exanthem?}:::decision D -->|Maculopapular, cephalocaudal| E[Measles]:::action D -->|Petechial, sandpaper texture| F[Scarlet fever]:::outcome D -->|Lacy reticular rash, slapped cheek| G[Erythema infectiosum]:::outcome D -->|Fine pink rash, postauricular nodes| H[Rubella]:::outcome ``` **High-Yield:** Unvaccinated status in rural India is a major risk factor for measles — it remains endemic in low-coverage areas. [cite:Park 26e Ch 8] [cite:Nelson Textbook of Pediatrics 21e Ch 245]