AIM Step 10 3rd Year MBBS MSK Module
Student Memory Support
Topic 17 — Viral, Fungal, Bacterial and Parasitic Skin Infections
High-yield memory reinforcement for rapid KMU revision.
1. High-Yield Flashcards
Tap each question to reveal the answer.
Where does herpes simplex virus remain latent after primary infection?
Sensory nerve ganglia.
Why does herpes zoster produce a unilateral dermatomal eruption?
Reactivated VZV travels from a sensory ganglion along its corresponding sensory nerve to the skin.
Which property allows dermatophytes to infect skin, hair and nails?
Ability to utilize keratin.
What clinical pattern suggests cutaneous candidiasis?
Moist erythematous macerated skin with peripheral satellite lesions.
Which organisms are commonly associated with impetigo?
Staphylococcus aureus and Streptococcus pyogenes.
Which anatomical layer is primarily involved in erysipelas?
Upper dermis and superficial lymphatics.
Which tissues are involved in cellulitis?
Deeper dermis and subcutaneous tissue.
What is a furuncle?
A deep infection of a hair follicle extending into surrounding dermis to form an abscess.
What is a carbuncle?
A larger deep inflammatory mass formed by interconnected furuncles.
What vascular event promotes tissue death in necrotizing soft-tissue infection?
Thrombosis of small vessels causing ischemia and tissue necrosis.
What is the antibacterial target of mupirocin?
Bacterial isoleucyl-tRNA synthetase.
How do azole antifungals impair fungal growth?
They inhibit ergosterol synthesis and disrupt fungal cell-membrane function.
Which enzyme is inhibited by terbinafine?
Squalene epoxidase.
What is the major antiviral target of acyclovir?
Viral DNA polymerase, resulting in reduced viral DNA synthesis.
Which host cell contains Leishmania in cutaneous leishmaniasis?
Macrophages.
2. Mnemonics
Mnemonic Title: Dermatophyte Genera
T-M-E
Meaning: Trichophyton — Microsporum — Epidermophyton.
Mnemonic Title: Bacterial Infection Depth
E-F-D
Meaning: Epidermis → impetigo; Follicle → folliculitis/furuncle; Deeper dermis → cellulitis.
Mnemonic Title: Major Antimicrobial Targets
M-A-T-A
Meaning: Mupirocin → isoleucyl-tRNA synthetase; Azoles → ergosterol synthesis; Terbinafine → squalene epoxidase; Acyclovir → viral DNA polymerase.
3. Memory Tables
Herpes Simplex vs Herpes Zoster
| Feature | Herpes Simplex | Herpes Zoster |
|---|---|---|
| Latency | Sensory ganglia | Sensory ganglia |
| Recurrence pattern | Localized grouped vesicles | Unilateral dermatomal vesicles |
| Pain | Local burning or pain | Prominent sensory nerve pain |
Common Bacterial Skin Infections
| Condition | Main Site | High-Yield Clue |
|---|---|---|
| Impetigo | Superficial epidermis | Crusted superficial erosions |
| Erysipelas | Upper dermis and lymphatics | Raised, well-defined erythema |
| Cellulitis | Deep dermis and subcutis | Diffuse, poorly defined swelling |
| Folliculitis | Hair follicle | Follicle-centered pustule |
| Abscess | Dermis/subcutaneous tissue | Localized pus-filled cavity |
4. Rapid Revision Points — Last-Minute Revision
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- Herpesviruses persist after primary infection because they establish latency in sensory ganglia.
- Dermatomal distribution links herpes zoster directly to sensory nerve reactivation.
- Dermatophytes infect keratinized tissues; Candida favors warm, moist and occluded surfaces.
- Tinea typically expands through superficial keratinized tissue with an active scaly margin.
- Impetigo is superficial, while cellulitis involves deeper dermis and subcutaneous tissue.
- Erysipelas is more superficial than cellulitis and therefore has a more sharply defined raised margin.
- Furuncle = one deep follicular abscess; carbuncle = interconnected furuncles.
- Rapid progression, severe pain and tissue discoloration should suggest necrotizing soft-tissue infection.
- Systemic antifungal treatment becomes important when dermatophyte infection is extensive or involves nails.
- Cutaneous leishmaniasis progresses slowly from papule or nodule to chronic ulcer and may heal with scarring.
KMU Exam Trap: Erysipelas is superficial and sharply demarcated; cellulitis is deeper and more poorly defined.
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5. Clinical Memory Hooks
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Recurrent lip vesicles after stress → HSV reactivation from sensory ganglion
Painful unilateral band of vesicles → VZV reactivation along a sensory dermatome
Moist skin-fold rash with peripheral lesions → Cutaneous candidiasis
Slowly enlarging exposed-skin papule becoming an ulcer → Cutaneous leishmaniasis with macrophage infection
6. High-Yield Exam Points
- ⭐ Herpes zoster results from reactivation of latent VZV in a sensory ganglion and produces a dermatomal eruption.
- ⭐ Dermatophytes have affinity for keratinized tissues: stratum corneum, hair and nails.
- ⭐ Necrotizing soft-tissue infection causes vascular thrombosis → ischemia → tissue necrosis.
- ⭐ Mupirocin inhibits bacterial isoleucyl-tRNA synthetase and therefore bacterial protein synthesis.
- ⭐ Azoles inhibit fungal ergosterol synthesis, while terbinafine inhibits squalene epoxidase.
- ⭐ Acyclovir inhibits viral DNA polymerase after preferential activation in herpesvirus-infected cells.
- ⭐ Leishmania survives and multiplies within macrophages, producing a slowly progressive cutaneous lesion that may ulcerate and scar.