Infectious Dermatology and Sexually Transmitted Infections
Classify by organism and depth
Bacterial: impetigo (Staphylococcus aureus; bullous form toxin-mediated), erysipelas (sharply demarcated, raised, febrile) versus cellulitis (ill-defined). Fungal: dermatophytes (tinea capitis, corporis, pedis, unguium), Malassezia, Candida. Infestation: scabies and lice. Viral: HSV, VZV, HPV, molluscum, enteroviruses.
Recognisable pictures
Scabies: nocturnal itch, finger webs, wrists, genital nodules, burrows; dermoscopy shows the delta-wing sign; treat the patient and all close contacts simultaneously. Herpes zoster: dermatomal vesicles, antivirals ideally within 72 hours, urgent ophthalmology review if ophthalmic branch. Tinea capitis: patchy alopecia with broken hairs and cervical nodes - needs oral therapy and KOH or Wood's lamp confirmation.
Venereology
Syphilis is the great imitator. Primary: painless indurated chancre. Secondary: copper-red maculopapular rash on palms and soles, condylomata lata, mucous patches, patchy alopecia. Latent and tertiary disease follow. Diagnose by dark-ground microscopy of a chancre early, serology later. Gonorrhoea: purulent discharge with Gram-negative intracellular diplococci. Chlamydia is frequently silent and needs NAAT. Genital herpes is a leading cause of genital ulceration.
Traps
- Palmoplantar rash with systemic symptoms = syphilis serology, not a "fungal" label.
- Scabies "treatment failure" is usually untreated contacts or reinfection.
- A negative early syphilis serology does not exclude infection - repeat at 6 and 12 weeks.
- Do not overlook partner notification, safeguarding, and HIV testing.
Rule of thumb
Any anogenital ulcer, any palmoplantar rash, and any newly diagnosed STI: screen for all STIs including HIV and syphilis.