Scabies crusted disease — DTM&H MCQ

Correct answer: A — Crusted scabies

Crusted scabies (option C) is correct: in an immunosuppressed host, the mite burden escalates massively because there is an inadequate host immune response to limit proliferation, producing thick hyperkeratotic crusted plaques on the hands and scalp that classically have minimal itch despite carrying an extremely high mite load. This form was historically called Norwegian scabies and is highly infectious, explaining why crowded shelter contacts develop typical pruritic papular scabies from brief exposure, whereas the index case himself is often paucisymptomatic. UKHSA guidance highlights that crusted scabies contains an estimated far higher mite density than ordinary scabies and that even transient contacts require assessment because of this increased infectivity. The combination of immunosuppression, hyperkeratotic plaques, and an outbreak of itchy papules in close contacts is the classic exam pattern for crusted scabies. Why the other options are wrong: E. Tungiasis: caused by the burrowing flea Tunga penetrans, producing painful nodular lesions on the feet, not thick crusted hand/scalp plaques, and it does not explain clustered pruritic papules in multiple contacts. C. Norwegian leprosy: not a real diagnostic entity; this distractor conflates the historical term Norwegian scabies with leprosy, which instead causes hypopigmented anaesthetic patches or nodules, not crusted itch-poor plaques with contact transmission. B. Psoriasis: produces well-demarcated silvery scaly plaques but is not contagious and would not cause pruritic papules in multiple shelter contacts. D. Cutaneous larva migrans: causes serpiginous, intensely itchy migrating tracks typically on the feet from walking on contaminated sand, not crusted plaques, and is not transmissible person-to-person. Key point: crusted (Norwegian) scabies in an immunosuppressed patient causes minimally itchy hyperkeratotic plaques but an enormous mite load that readily transmits classic pruritic scabies to close contacts.

Reference: UK Health Security Agency, Management of scabies cases and outbreaks in communal residential settings (2023), gov.uk