A physician-validated, board-style question from the Active Transport QBank. Try it, then check the reasoning for every option.
A 13-month-old girl is brought to the physician because of a pruritic rash for 2 days. The girl's mother says she noticed a few isolated skin lesions on her trunk two days ago that appear to be itching. The girl received her routine immunizations 18 days ago. Her mother has been giving her ibuprofen for her symptoms. The patient has no known sick contacts. She is at the 71st percentile for height and the 64th percentile for weight. She is in no acute distress. Her temperature is 38.1°C (100.6°F), pulse is 120/min, and respirations are 26/min. Examination shows a few maculopapular and pustular lesions distributed over the face and trunk. There are some excoriation marks and crusted lesions as well. Which of the following is the most likely explanation for these findings?
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A
Antigen contact with presensitized T-lymphocytesIncorrect. Type IV hypersensitivity (presensitized T cells) causes contact dermatitis or PPD reaction — not maculopapular/pustular vesicular rash 18 days after live-virus vaccination.
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B
Reactivation of virus dormant in dorsal root ganglionIncorrect. VZV reactivation (zoster) follows latent infection and presents in a dermatomal pattern, typically years later — not 18 days after primary immunization.
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C
Crosslinking of preformed IgE antibodiesIncorrect. Type I hypersensitivity (IgE cross-linking) causes urticaria or anaphylaxis within minutes to hours of vaccine — too quick and wrong morphology for this 18-day-later rash.
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D
Replication of the attenuated vaccine strainCorrect. Mild vesicular rash 5–26 days after varicella vaccination represents replication of the live-attenuated vaccine strain — a known, self-limited reaction in ~3–5% of recipients.
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E
Immune-complex deposition in dermal vesselsIncorrect. Immune-complex (type III) deposition causes serum sickness or leukocytoclastic vasculitis with palpable purpura — not the pustular vesicular rash following live-virus vaccination, which represents vaccine-strain replication.
↑ Tap an answer to reveal the reasoning
Answer: D. A pruritic rash with maculopapular and pustular lesions, plus low-grade fever, in a 13-month-old who received routine vaccinations 18 days earlier is consistent with mild varicella from the live-attenuated varicella vaccine. The MMR-V (or separate varicella) vaccine is given at 12–15 months; vaccine-strain varicella develops in ~3–5% of recipients, typically 5–26 days after immunization, with a sparse maculopapular/pustular rash and fever — much milder than wild-type chickenpox.
The mechanism is REPLICATION OF THE ATTENUATED VACCINE STRAIN. The vaccine-strain virus replicates locally and produces a brief, self-limited varicelliform eruption. The disease is mild, self-limited, and typically does not require treatment, though acyclovir can be considered in immunocompromised patients.
Distractors: 'Antigen contact with presensitized T-lymphocytes' = type IV delayed hypersensitivity (contact dermatitis), not a vesicular rash 18 days post-immunization. 'Reactivation of virus dormant in dorsal root ganglion' = herpes zoster (shingles), which appears years after primary infection and is dermatomal — not in a 13-month-old just immunized. 'Crosslinking of preformed IgE antibodies' = type I hypersensitivity (urticaria, anaphylaxis), which would appear within minutes to hours of vaccination, not 18 days later.
**Why each option:**
**A.** Type IV hypersensitivity (presensitized T cells) causes contact dermatitis or PPD reaction — not maculopapular/pustular vesicular rash 18 days after live-virus vaccination.
**B.** VZV reactivation (zoster) follows latent infection and presents in a dermatomal pattern, typically years later — not 18 days after primary immunization.
**C.** Type I hypersensitivity (IgE cross-linking) causes urticaria or anaphylaxis within minutes to hours of vaccine — too quick and wrong morphology for this 18-day-later rash.
**D.** Correct. Mild vesicular rash 5–26 days after varicella vaccination represents replication of the live-attenuated vaccine strain — a known, self-limited reaction in ~3–5% of recipients.