## Distinguishing Measles from Rubella: Koplik Spots **Key Point:** Koplik spots are the pathognomonic early sign of measles, appearing 2–3 days before the rash and disappearing as the rash emerges. They are NOT seen in rubella. ### Koplik Spots: Pathognomonic Feature - **Appearance:** Small white spots with red halos on the buccal mucosa opposite the molars - **Timing:** Prodromal phase (days 2–4 of illness) - **Significance:** Appears before the measles rash; diagnostic even before maculopapular rash develops - **Histology:** Epithelial necrosis and inflammatory infiltrate - **Duration:** Fade as the rash appears (usually by day 4–5) ### Comparison Table: Measles vs. Rubella | Feature | Measles | Rubella | | --- | --- | --- | | **Koplik spots** | Present (pathognomonic) | Absent | | **Fever** | High (39–40°C), 3–4 days | Mild (37–38°C), 1–2 days | | **Cough** | Prominent (3 C's: cough, coryza, conjunctivitis) | Minimal or absent | | **Rash onset** | Day 4–5 of illness | Day 1–2 of illness | | **Rash character** | Maculopapular, confluent, blanching | Maculopapular, discrete, non-blanching | | **Lymphadenopathy** | Posterior cervical (rare) | Suboccipital & posterior cervical (common) | | **Complications** | Pneumonia, encephalitis, SSPE | Congenital defects (if in pregnancy) | **High-Yield:** Koplik spots are the **only pathognomonic sign** of measles and appear in the prodromal phase — before the rash. Their presence confirms measles diagnosis even before the characteristic maculopapular rash appears. **Clinical Pearl:** The "3 C's" of measles prodrome (Cough, Coryza, Conjunctivitis) + high fever + Koplik spots = measles until proven otherwise. Rubella, by contrast, has minimal systemic symptoms and NO Koplik spots. **Mnemonic: "KOPLIK = Measles Only"** — Remember: Koplik spots are Only Present in measles, not rubella or other viral exanthems.
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