A 4-year-old boy is brought to the clinic by his mother with fever and a rash. The patient’s mother says his symptoms started 1 week ago with the acute onset of fever and a runny nose, which resolved over the next 3 days. Then, 4 days later, she noted a rash on his face, which, after a day, spread to his neck, torso, and extremities. The patient denies any pruritus or pain associated with the rash. No recent history of sore throat, chills, or upper respiratory infection. The patient has no significant past medical history and takes no medications. The vital signs include: temperature 37.2°C (99.9°F) and pulse 88/min. On physical examination, there is a maculopapular rash on his face, torso, and extremities, which spares the palms and soles. The appearance of the rash is shown in the exhibit (see image below). Which of the following would most likely confirm the diagnosis in this patient?

ELISA for parvovirus B-19 IgM and IgG antibodies

Assay for IgM and IgG against measles virus

Serology for human herpesvirus-6 IgM antibodies

ELISA for IgM antibodies against Rubella virus

An 8-year-old boy presents with recurrent infections including multiple episodes of pneumonia and diarrhea. He reports difficulty seeing in the dark. Physical examination reveals white patches on the sclera and conjunctival dryness. What is the most likely cause of these findings?

Spinocerebellar ataxia (SCA) type 1

A 10-year-old girl with a rash is brought to the clinic by her mother. The patient’s mother says that the onset of the rash occurred 2 days ago. The rash was itchy, red, and initially localized to the cheeks with circumoral pallor, and it gradually spread to the arms and trunk. The patient’s mother also says her daughter had been reporting a high fever of 39.4°C (102.9°F), headaches, myalgia, and flu-like symptoms about a week ago, which resolved in 2 days with acetaminophen. The patient has no significant past medical history. Her vital signs include: temperature 37.0°C (98.6°F), pulse 90/min, blood pressure 125/85 mm Hg, respiratory rate 20/min. Physical examination shows a symmetric erythematous maculopapular rash on both cheeks with circumoral pallor, which extends to the patient’s trunk, arms, and buttocks. The remainder of the exam is unremarkable. Laboratory findings are significant for a leukocyte count of 7,100/mm3 and platelet count of 325,000/mm3. Which of the following is the next best step in the management of this patient?

Discharge home, saying that the patient may immediately return to school

Administer intravenous immunoglobulin (IVIG)

Discharge home, saying that the patient may return to school after the disappearance of the rash

Discharge home with instructions for strict isolation from pregnant women until disappearance of the rash

A 9-year-old male presents to your office with an indurated rash on his face. You diagnose erythema infectiosum. Which of the following is characteristic of the virus causing this patient's disease?

Non-enveloped virus with single-stranded DNA

Enveloped virus with single-stranded RNA

Non-enveloped virus with double-stranded DNA

Enveloped virus with single-stranded DNA

Enveloped virus with double-stranded DNA

Common childhood exanthems for USMLE Step 3: High-Yield Pediatrics Lessons

Exanthem Showdown - The Rash Pack

Child with maculopapular rash

Feature	Measles (Rubeola)	Rubella (German Measles)	Erythema Infectiosum (Fifth Dis.)	Roseola Infantum (Sixth Dis.)	Varicella (Chickenpox)
Virus	Paramyxovirus	Togavirus	Parvovirus B19	HHV-6, HHV-7	Varicella-Zoster Virus (VZV)
Incubation	10-14 days	14-21 days	4-14 days	~10 days	10-21 days
Prodrome	Cough, Coryza, Conjunctivitis (3 Cs), high fever	Low-grade fever, posterior auricular/suboccipital lymphadenopathy	Mild fever, malaise	High fever (>39.5°C) for 3-5 days, child appears well	Fever, malaise, pharyngitis
Exanthem	Erythematous maculopapular, cephalocaudal spread, coalescing. Spares palms/soles.	Maculopapular, rapid cephalocaudal spread (faster than measles), non-confluent.	Phase 1: "Slapped cheek" rash. Phase 2: Erythematous macular rash. Phase 3: Reticular, lace-like rash on trunk/extremities.	Macular/maculopapular rash appears after fever defervescence. Starts on trunk, spreads to face/limbs.	Vesicular rash on erythematous base ("dew drop on a rose petal"). Pleomorphic (all stages present). Pruritic.
Pathognomonic	Koplik's spots (bluish-white spots on buccal mucosa)	Forchheimer spots (petechiae on soft palate)	-	Nagayama spots (ulcers on uvulopalatoglossal junction)	-
Complications	SSPE, otitis media, pneumonia (giant cell), encephalitis	Congenital Rubella Syndrome (CRS), arthritis	Aplastic crisis (in sickle cell), hydrops fetalis	Febrile seizures	Secondary bacterial infection, pneumonia, encephalitis, Reye's syndrome

Special Cases - The Red Alerts

Congenital Rubella Syndrome (CRS): Classic Gregg's Triad:
- Cataracts (or glaucoma)
- Sensorineural deafness
- Patent Ductus Arteriosus (PDA)
Atypical Measles: Occurs in individuals vaccinated with older killed-virus vaccines (pre-1968).
- Presents with high fever, headache, and a rash that starts distally (wrists/ankles) and moves centripetally. Spares the face.
Kawasaki Disease (KD): A crucial vasculitis differential for rash + fever.
- Fever for ≥5 days PLUS 4 of 5 criteria: 📌 CRASH & Burn (Fever)
  - Conjunctivitis (bilateral, non-exudative)
  - Rash (polymorphous)
  - Adenopathy (cervical, often unilateral >1.5 cm)
  - Strawberry tongue / mucosal changes
  - Hand/foot changes (erythema, edema, desquamation)

⭐ The most feared complication of Kawasaki Disease is coronary artery aneurysm, hence the importance of early diagnosis and IVIg treatment.

Diagnosis Dash - Rash Algorithm

⭐ Nagayama spots (papular enanthem on uvula/soft palate) are seen in ~60% of Roseola Infantum cases, appearing as the fever breaks.

Measles (Rubeola): Prodrome of cough, coryza, conjunctivitis (3Cs) and pathognomonic Koplik's spots before a cephalocaudal rash.

Erythema Infectiosum (Fifth Disease): Caused by Parvovirus B19, presenting with a "slapped-cheek" appearance followed by a lacy, reticular rash.

Roseola Infantum (Sixth Disease): Caused by HHV-6; a high fever precedes a rash that appears as the fever subsides.

Rubella: Distinguished by prominent postauricular and suboccipital lymphadenopathy; a major teratogen.

Chickenpox (Varicella): Characterized by a pleomorphic rash (lesions in various stages) appearing in crops.

Kawasaki Disease: The most feared complication is coronary artery aneurysm; look for prolonged fever and mucosal changes.

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