# Viral Exanthems

> Viral exanthems in NEET-PG Dermatology: Koplik spots, Forchheimer spots, roseola after fever, slapped cheek parvovirus B19 and varicella crops.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/viral-exanthems
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Viral Exanthems", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/viral-exanthems

## Direct answer

The classic viral exanthems are separated by their prodrome, enanthem and rash-day arithmetic. Measles runs 3-4 days of the three Cs — cough, coryza, conjunctivitis — with Koplik spots (grain-of-salt white papules on the buccal mucosa) on day 2-3, and only then a brick-red maculopapular rash that starts behind the ears and hairline and descends, lasting about six days with fine desquamation. Rubella is milder: soft-palate Forchheimer spots, tender postauricular and suboccipital nodes, and a pink rash that appears on day 1-3 and clears in three days, with arthritis in adult women and devastating congenital disease. Roseola infantum (human herpesvirus 6) fevers for 3-5 days and then, precisely as the fever breaks, drops a pale maculopapular rash on the trunk. Erythema infectiosum (parvovirus B19) gives slapped-cheek erythema succeeded by a lacy reticulate eruption. Varicella shows crops of dew-drop vesicles in different stages of evolution simultaneously, distributed centripetally.

## What you must remember

- **Measles timeline:** incubation about 10-12 days; Koplik spots precede the rash and disappear as it erupts; infectivity runs from 4 days before to 4 days after rash onset — the quarantine arithmetic examiners love.
- **Measles complications and vitamin A:** otitis media, pneumonia (the killer in Indian children), diarrhoea, and subacute sclerosing panencephalitis 7-10 years later; WHO gives two doses of vitamin A 24 hours apart (100,000 IU at 6-11 months, 200,000 IU at 12 months and above) in deficiency-endemic regions.
- **India's immunisation frame:** measles-rubella vaccine at 9-12 months with a second dose at 16-24 months under UIP — rubella's congenital syndrome (deafness, cataract, cardiac defects) is the reason MR replaced measles-only vaccine.
- **Rubella landmarks:** Forchheimer spots on the soft palate, postauricular/suboccipital lymphadenopathy, arthritis of adult women, and a fetal risk highest in the first 16 weeks of gestation.
- **Roseola (exanthem subitum):** HHV-6, infants under 2; high fever, well-looking child, febrile seizures, then rash on defervescence — "the rose follows the fall of the fever" — with Nagayama palatal spots.
- **Erythema infectiosum (B19):** slapped cheek then lacy eruption; aplastic crisis in haemoglobinopathy children (reticulocytopenia), hydrops fetalis in early pregnancy, and small-joint arthritis in adult women.
- **Varicella mechanics:** successive crops over 3-4 days mean vesicles, pustules and crusts coexist ("synchrony of stages" is asynchronous — unlike smallpox's uniform stage); contagious from 48 hours before rash until all lesions crust; aspirin is avoided (Reye syndrome); oral aciclovir within 24 hours for adults, pregnancy and immunocompromise.
- **Zoster clue in a child or young adult:** think of underlying immunosuppression, including HIV screening in the Indian context.

## Sorting a fever-plus-rash casualty

A 3-year-old reaches the OPD on day 4 of fever with red eyes, a barking cough and a blotchy face. Step one: look in the mouth — Koplik spots on the buccal mucosa close the argument for measles before a single rash sentence is written. Step two: date the public-health obligations — the child was infectious from the day before the rash and remains so for four days after its onset, so vitamin A two doses 24 hours apart are given, complications are explained, and unvaccinated household contacts under 9 months are assessed. Contrast the next cot: an 11-month-old, fever 39.5 degrees for three days, playful between spikes, has just defervesced and now sports rose-pink trunk macules that blanch — roseola, and the correct management is reassurance, not antibiotics, a message worth practising because these infants receive more unnecessary co-amoxiclav than almost any other group.

The third vignette: a schoolteacher's cheeks went scarlet "like sunburn" and three days later her arms carry a lacy erythema. She is 14 weeks pregnant, which converts a benign childhood illness into an urgent parvovirus B19 IgM and IgG — past infection is reassuring, susceptibility needs monitoring, because B19 hydrops risk concentrates in the second trimester.

## Where the exam sets the discriminator

Almost every exanthem stem reduces to two variables: rash-fever synchrony and the enanthem. Fever with the rash points to measles, rubella and B19; fever falling as the rash appears is roseola. Koplik nails measles, Forchheimer suggests rubella, Nagayama belongs to roseola. The coexistence-of-stages rule separates chickenpox (all stages at once) from historical smallpox. Know the danger attachments: B19 with haemoglobinopathy and pregnancy, varicella with aspirin (Reye) and immunocompromise (IV aciclovir), measles with vitamin A.

## Frequently asked questions

### Which enanthem is pathognomonic of measles?

Koplik spots — bluish-white "grains of salt" on an erythematous buccal mucosa, seen on days 2-3, fading as the rash erupts.

### When is a patient with measles infectious?

From about 4 days before to 4 days after the onset of the rash, which defines isolation and contact-prophylaxis windows.

### Why does roseola infantum present with rash after fever?

HHV-6 fever lasts 3-5 days, and the pale maculopapular eruption appears within 24-48 hours of defervescence — fever-rash dissociation is its signature.

### What complication of parvovirus B19 occurs in a child with sickle cell disease?

A transient aplastic crisis with reticulocytopenia and profound anaemia, because B19 suppresses erythroid precursors.

### How is the rash of chickenpox distinguished morphologically?

Successive crops mean vesicles, pustules and crusts coexist at different stages ("dew drops on rose petals"), distributed centripetally with predilection for the trunk.