Molluscum contagiosum and viral warts — short-answer questions

This stem concerns two of the commonest benign viral skin infections of childhood in an atopic child, illustrating clinical diagnosis, the expectant-versus-active treatment decision, and the role of evidence in counselling anxious parents. [2]

Question 1 (10 marks)

a) Give the two diagnoses, the causative viruses, and the key clinical features that identify each. (5 marks) [2]

The axillary lesions are molluscum contagiosum, caused by Molluscum contagiosum virus (MCV), a brick-shaped double-stranded DNA poxvirus of the genus Molluscipoxvirus. The identifying features are the small, smooth, dome-shaped, pearly, flesh-coloured papules, each with a glistening central dell (umbilication), clustered in the flexures and gradually increasing in number over months. The thumb lesion is a common viral wart (verruca vulgaris), caused by human papillomavirus (commonly HPV-2, 27 or 57); the identifying features are the rough, firm, hyperkeratotic surface with tiny black dots representing thrombosed capillary loops. [2]

His atopic dermatitis is relevant because the epidermal barrier defect of eczema increases susceptibility to both infections, and his weekly swimming is a recognised contributor to wart acquisition through moist, macerated skin and shared surfaces. [2]

b) Outline the pathophysiology of each and explain why both are self-limiting. (5 marks) [2]

In molluscum, the poxvirus infects lower epidermal keratinocytes; infected cells swell with large eosinophilic intracytoplasmic inclusions called molluscum (Henderson-Patterson) bodies. As these cells migrate upward, the overlying stratum corneum degenerates centrally and the viral core is extruded through the umbilicated dell. The virus evades local immunity, which explains the long indolent course until cell-mediated recognition switches on, often heralded by an inflammatory halo around the lesions. [2]

In warts, human papillomavirus enters basal keratinocytes through micro-trauma and drives keratinocyte proliferation and abnormal keratinisation, producing koilocytes (ballooned cells with pyknotic nuclei), hyperkeratosis, acanthosis and elongated rete ridges. Both conditions resolve through a cell-mediated (Th1-type) immune response, which is why lesions may persist for months and then vanish — the biological basis for expectant management. [2]

Question 2 (10 marks)

a) How would you counsel this mother about prognosis and the expectant-versus-active treatment decision for the molluscum? (6 marks) [3]

I would explain that molluscum is benign and self-limiting, with a median time to resolution of about 13 months, roughly half of children clear within one year, and the great majority within two years, with a measurable but modest effect on quality of life. Expectant, conservative care — reassurance, no sharing of towels, avoiding scratching and picking, and covering lesions — is the evidence-based default. I would explicitly address her fear of scarring, explaining that the lesions themselves rarely scar, whereas over-aggressive destructive treatment can. [3]

If the lesions are genuinely bothersome or spreading, I would offer an active option chosen with her: in-office cantharidin 0.7 per cent topical solution, or a home-applied topical such as potassium hydroxide 10 per cent or berdazimer 10.3 per cent gel (the first home prescription topical, shown effective in three randomised trials). I would not offer imiquimod, because randomised trials showed no benefit over placebo. The decision should weigh the modest efficacy and local side-effects against the natural history. [5]

b) Outline your management of the thumb wart, citing the key trial evidence. (4 marks) [8]

First-line active treatment for a common wart is salicylic acid 12 to 26 per cent paint or plaster, applied daily to the soaked and pared wart with protection of surrounding skin. If it is persistent or bothersome, cryotherapy with liquid nitrogen every two to four weeks is the next step. The Cochrane review found salicylic acid modestly superior to placebo but the evidence for cryotherapy superiority weak, and the randomised EVerT trial found no clear clinical or cost-effectiveness advantage of cryotherapy over salicylic acid for plantar warts. I would set expectations that several weeks of treatment are usual, and that watchful waiting is also a legitimate choice in a young child given the high spontaneous clearance rate. [8]