Cutaneous Infections and Infestations
Organism, host, and pattern
Infections are tested through pattern recognition plus one confirmatory test. Know the classic morphology for each organism and the bedside test that proves it.
Concepts to master
- Dermatophytes: annular scaly plaques with central clearing and an active border; KOH shows septate branching hyphae. Tinea capitis in children causes patches of alopecia with broken hairs; KOH or culture is required before systemic therapy.
- Candida: beefy-red intertriginous plaques with satellite pustules.
- Pityriasis versicolor: hypopigmented or hyperpigmented macules on the trunk; KOH shows short hyphae with spores (spaghetti and meatballs).
- Bacterial: impetigo with honey-colored crust; erysipelas with a raised, sharply demarcated border; cellulitis without a sharp border.
- Viral: grouped vesicles on an erythematous base for herpes simplex and zoster; dermatomal distribution for zoster; umbilicated papules for molluscum contagiosum.
- Infestations: scabies with burrows in web spaces, wrists, and genitalia, intense nocturnal itch, and a positive mineral oil preparation; lice with nits on hair shafts.
Traps
A negative KOH does not exclude tinea - poor scraping technique is common. Do not diagnose zoster without considering zosteriform herpes simplex, which can recur in the same dermatome. Scabies in infants and the elderly can present atypically with crusted or bullous lesions. Impetigo can be bullous (toxin-mediated) or nonbullous, and both are highly contagious.
Worked example
A child with an itchy scalp, patchy hair loss, and occipital lymphadenopathy. Diagnosis: tinea capitis. Confirm with KOH or fungal culture of plucked hairs or scale, then treat with oral antifungal therapy - topical agents do not penetrate the follicle adequately.