USMLE MSK 15: Skin Infections

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  / lymed   Welcome to LY Med, where I go over everything you need to know for the USMLE STEP 1, with new videos every day.

Follow along with First Aid, or with my notes which can be found here: https://www.dropbox.com/sh/ces0j7obod...

This video is going to be on skin infections. Most of the skin infections you'll see will be from two causative agents: Staph Aureus and Strep. Now strep can cause a ton of things, like pharyngitis, scarlet fever, post-strep glomerulonephritis, and rheumatic fever. However, one thing to know is that skin infections from strep can lead to post-strep GN, but it does not cause rheumatic fever! Just something to keep in mind.

Now let's talk about some skin infections:
Impetigo: this is a superficial skin infection that can cause crusting. Often called honey crusting due to it's yellowish crust. It can also be bullous, and if there are bullae, it's often staph. Now, the infection can invade deeper and cause cellulitis. This is a deep invasion into the dermis and subcutaneous tissue - the fat. You'll see erythematous skin without clear margins. If there ARE clear margins, then you're thinking erysipelas. This is a well demarcated erythema and usually strep pyogenes. It may be well demarcated due to it's invasion into the lymphatic drainage.

Moving on, we have a special skin condition called staphlococcal scalded skin syndrome: This is when Staph releases exfoliative A/B exotoxin. These are proteases that act on desmoglein and destroy attachment. What else attacks desmoglein? That's right - pemphigus vulgaris. Side note: anytime you think of skin separation, always think of SJS and TEN. However those were due to separation of dermal and epidermal layers.

Next up is necrotizing fasciitis. This can be from staph and strep, but it can also be from anaerobic bacteria like Clostridium perfringens. These bacteria produce a lot of gas and can infect multiple sites. Thus they present as skip lesions (non-continuous) and the skin will have crepitus from the methane/CO2 production! Also known as gas gangrene.

Those are all your bacterial infections, let's talk about viral infections. A common one is herpes - HSV1 and HSV2. HSV1 is commonly thought of as the cause of oral lesions (cold sores) and HSV2 causes genital lesions. Another common viral lesion is molluscum contagiosum. This is NOT chickenpox. Their lesions have an indentation right in the body. We call these umbilicated lesions. This is the easiest way to differentiate them!

Well what causes chickenpox? This is from varicella zoster virus. This virus causes chickenpox (varicella) and can reactivate and cause shingles (zoster). Shingles causes vesicular lesions often in the trigeminal region and under the breasts. It can lead to neuropathic pain.

Next up is hairy leukoplakia which is caused by EBV. This leads to white plaques that can't be scraped off.

Let's move on to fungus and parasites. A common one is scabies - a superficial infection from the parasite Sarcoptes scabiei. This bug leaves eggs and feces and causes severe pruritis from a type IV hypersensitivity. It leads to thin raised lines in the skin. In immunosuppressed patients, it can lead to severe dissemination and crusting. You can pick it up with mineral oil prep and treatment is with topical permethrin.

Fungal infections include dermatophytes. This is a superficial fungal infection that likes to eat the keratin of the skin. Confirmation is with KOH prep. Treatment is with antifungals.

Last is tinea versicolor. This is due to the agents malassezia globosa or malassezia furfur. It causes white hypopigmentation patches due to the fungal production of azelaic acid which has a bleaching effect. KOH prep reveals spaghetti and meatball appearance of hypae and yeast respectively. You can treat it with antifungals or selenium like Selsun Blue!