Infectious Diseases of the Skin

Bacterial Infections - Skin's Bacterial Battles

• Impetigo: S. aureus, Group A Strep (GAS). Superficial.
  • Non-bullous: Honey-colored crusts. Most common.
  • Bullous: S. aureus (exfoliative toxins A/B). Flaccid bullae.
• Ecthyma: Deeper, ulcerative impetigo; "punched-out" ulcers.
• Folliculitis: S. aureus. Hair follicle inflammation. Furuncle (deep), Carbuncle (multiple).
• Erysipelas: GAS. Superficial dermis, lymphatics. Sharp, raised, erythematous border.
• Cellulitis: S. aureus, GAS. Deeper dermis, subcutis. Indistinct borders.
• Staphylococcal Scalded Skin Syndrome (SSSS): Neonates/infants. Exfoliative toxins. Nikolsky sign +.

  ⭐ Nikolsky sign (gentle rubbing causes skin exfoliation) is positive in SSSS.

• Leprosy (Hansen's Disease): M. leprae. Spectrum: Tuberculoid (TT) ↔ Lepromatous (LL). Nerve damage, anesthetic patches.
• Cutaneous TB: M. tuberculosis.
  • Lupus vulgaris: Persistent; red-brown "apple-jelly" nodules (diascopy).
  • Scrofuloderma: Direct extension from underlying TB (e.g., lymph node).

Fungal Infections - Myco Mayhem Moshpit

• Dermatophytoses (Tinea):
  • Agents: Trichophyton, Microsporum, Epidermophyton.
  • Invade keratinized tissue (skin, hair, nails).
  • Dx: KOH mount (septate hyphae); Wood's lamp (Microsporum fluoresces green).
• Yeast Infections:
  • Candida albicans: Pseudohyphae, budding yeasts. Affects oral, genital, intertriginous areas.
  • Malassezia furfur (Pityriasis versicolor): KOH: "spaghetti & meatballs" (short hyphae & spores). 📌 Mnemonic: "Meatballs and spaghetti, a Malassezia party!"
• Subcutaneous Mycoses:
  • Sporotrichosis (Sporothrix schenckii): Cigar-shaped yeasts; asteroid bodies (Splendore-Hoeppli).
  • Chromoblastomycosis: Sclerotic bodies ("copper pennies"/Medlar bodies).
  • Mycetoma: Tumefaction, sinuses, grains.

⭐ In Chromoblastomycosis, the characteristic sclerotic bodies (Medlar bodies or "copper pennies") are diagnostic, representing the fungus in its tissue form with internal septations.

Viral Infections - Viral Skin Vexations

• Herpes Simplex (HSV):
  • Painful vesicles on erythematous base; HSV-1 (oral), HSV-2 (genital).
  • Tzanck smear: Multinucleated giant cells (MNGs), Cowdry A inclusions.
• Varicella-Zoster (VZV):
  • Varicella (Chickenpox): "Dew drops on a rose petal" vesicles; centripetal.
  • Zoster (Shingles): Unilateral, dermatomal painful vesicles; risk of Post-Herpetic Neuralgia (PHN).
  • MNGs on Tzanck.
• Human Papillomavirus (HPV):
  • Warts (Verrucae): Vulgaris, plana, plantaris, anogenital (condyloma acuminata).
  • Histology: Koilocytes (perinuclear halo).
• Molluscum Contagiosum:
  • Poxvirus. Pearly, dome-shaped, umbilicated papules.
  • Intracytoplasmic Henderson-Paterson bodies (molluscum bodies).
• Measles (Rubeola):
  • Koplik spots (buccal mucosa). Maculopapular rash (starts face → trunk).
  • Warthin-Finkeldey giant cells.
• Hand, Foot, and Mouth Disease (HFMD):
  • Coxsackie A virus. Vesicles on hands, feet, oral mucosa.

⭐ Tzanck smear showing multinucleated giant cells is characteristic of HSV and VZV infections.

Parasitic & Protozoal - Itch & Scratch Saga

• Scabies: Sarcoptes scabiei. Intense nocturnal pruritus. Burrows (interdigital, wrists). Dx: microscopy (mites, eggs). Rx: Permethrin 5%, Ivermectin. Crusted scabies in immunocompromised.
• Pediculosis: Pediculus humanus (capitis, corporis), Pthirus pubis. Pruritus. Nits on hair. Rx: Permethrin 1%.
• Cutaneous Larva Migrans: Animal hookworm larvae. Serpiginous, pruritic tracts. Rx: Albendazole.
• Leishmaniasis (Cutaneous): Leishmania spp. Sandfly vector. Papule → nodule → painless ulcer (raised border). Dx: LD bodies.

  ⭐ PKDL (Post-Kala-Azar Dermal Leishmaniasis) presents with hypopigmented macules/papules/nodules, often facially, after visceral leishmaniasis treatment.

High‑Yield Points - ⚡ Biggest Takeaways

• Tzanck smear reveals multinucleated giant cells in Herpes infections.
• KOH mount is crucial for diagnosing fungal infections, showing hyphae/spores.
• Leprosy diagnosis involves AFB staining for Mycobacterium leprae in skin biopsies.
• Molluscum contagiosum is characterized by Henderson-Paterson bodies.
• Granuloma inguinale shows Donovan bodies (safety pin appearance) within macrophages.
• Scabies diagnosis involves identifying mites, eggs, or scybala on skin scrapings.
• Koilocytes are pathognomonic for HPV infections like warts (verrucae).