A three-year-old boy presented with a vesicular rash on the right flank and right buttock (see accompanying figure). There was no associated fever. The child was delivered vaginally after a pregnancy notable for maternal varicella at 10 weeks' gestation. Apgar scores were 7 and 9 at birth. He was breastfed for four months. His past health was unremarkable. He was not known by the parents to have chickenpox. Except for varicella vaccination, his immunizations were otherwise up-to-date. There was no recent exposure to infectious diseases. He did not have any known allergies.
The answer is B: herpes zoster. Herpes zoster (shingles) is caused by reactivation of varicella-zoster virus from a dorsal root ganglion to a cutaneous nerve and the adjacent skin. Herpes zoster is rare before 10 years of age.1 The incidence increases with age and rises sharply after 50 years of age. In general, the younger a child develops chickenpox, the greater the likelihood that herpes zoster will develop in childhood or early adulthood.1 Approximately 2 percent of children who were exposed to varicella-zoster virus in utero can develop inapparent chickenpox.2,3 In young children, herpes zoster often occurs in areas supplied by the cervical and sacral dermatomes, rather than in the lower thoracic and upper lumbar dermatomes, which are the characteristic sites of herpes zoster in adults.1 An area of erythema may precede the development of grouped vesicles. The eruption may be preceded or accompanied by low-grade fever, localized pain, hyperesthesias, and pruritus. In immunocompromised patients, herpes zoster can become disseminated, with lesions appearing outside the primary dermatomes and with visceral involvement.
The individual lesions of varicella are similar to herpes zoster, but the lesions of varicella are more widely distributed. They tend to be centrifugal, do not cluster in a specific dermatome, and occur in crops.
Molluscum contagiosum often presents in early childhood with clusters of discrete, pearly, flesh-colored, umbilicated papules. In contrast to herpes zoster, the lesions are not pruritic or painful, and vesicles are not seen.
Scabies presents as an intensely pruritic rash. Lesions are characteristically found on the wrist and intertriginous areas, including the diaper area in young children. Thread-like burrows are the classic sign of scabies.
The lesions of juvenile xanthogranuloma are dome-shaped, yellow, pink, orange, or brown nodules that vary in size from a few millimeters to 4 cm in diameter. They are usually present at birth or appear within the first six to nine months of life. Affected children are otherwise normal and have normal lipid levels.