Question:

An eight-year-old boy presented with three-day's history of flaccid blisters over his left upper arm and right calf. The blisters quickly ruptured and became superficial round erosions, some covered with crusting. The boy's younger sister has suffered from a similar condition a few days before and has fully recovered.

Answer: A. Bullous impetigo

While impetigo contagiosa (non-bullous impetigo) can be caused by both Streptococcal pyogenes and Staphylococcal aureus, bullous impetigo is always caused by coagulase positive Staphylococcus aureus. It presents with flaccid blisters that evolve into superficial erosions with either a shiny surface or covered with crust. The blisters are the results of a staphylococcus exotoxin (exfoliatin or epidermolytic toxin A or B). Blister fluid or the lifted edge of crusted plaques should be swabbed to send for culture and sensitivity. Empirical treatment with dicloxacillin, cloxacillin, combination of amoxicillin and clavulanic acid or a cephalosporin are usually effective.

Herpes simplex infections usually present with small grouped vesicles on an erythematous base. Also the sites of left arm and right calf are very uncommon in herpes simplex. Chickenpox lesions are denser on the trunk and are characterised by lesions in varying stages (2-4mm macules, fragile vesicles then pustules) present simultaneously. Secondary infection by S. aureus and S. pyogenes are not uncommon in chickenpox. Chronic bullous dermatosis of childhood is very rare and presents with vesicles and bullae in a "cluster of jewels" pattern with perioral and genital predilection.