Cutaneous Infections and Infestations
Scope
Expect questions across bacterial (impetigo, cellulitis, erysipelas, folliculitis, staphylococcal scalded skin syndrome, erythrasma), viral (herpes simplex, varicella-zoster, HPV, molluscum, measles), fungal (dermatophytes, Malassezia, Candida, deep mycoses), mycobacterial (tuberculosis, leprosy spectrum), parasitic (scabies, lice, leishmaniasis), and sexually transmitted infections (syphilis stages, gonorrhoea, chlamydia, HSV, HPV).
Key concepts
- Organism–lesion correlation: dermatophytes cause annular scaly plaques with central clearing; Candida favours moist intertriginous sites; Malassezia causes pityriasis versicolor with hypopigmentation.
- Leprosy is a spectrum (tuberculoid to lepromatous) driven by cell-mediated immunity; know the reactions.
- Syphilis is the great mimicker: chancre (primary), maculopapular palmoplantar rash (secondary), gumma (tertiary); serology interpretation matters.
- Scabies: burrows, nocturnal itch, web spaces; treatment of contacts is essential.
Traps
- Assuming all annular lesions are fungal - annular erythema, granuloma annulare, and pityriasis rosea mimic tinea.
- Missing palmoplantar involvement in secondary syphilis.
- Confusing staphylococcal scalded skin syndrome (superficial split, Nikolsky positive) with toxic epidermal necrolysis.
- Forgetting that HPV types 16 and 18 carry oncogenic risk.
Rule of thumb
For any suspected infection, identify the compartment (stratum corneum, epidermis, dermis, subcutis) and the host immune status; those two variables predict organism and severity.