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Bacterial Skin Infections
These three are often tested against each other. The win is recognizing depth, borders, and systemic features, then choosing topical vs oral vs IV.
Cellulitis
- Acute bacterial infection of the dermis and subcutaneous tissue
- Most commonly caused by Strep pyogenes or Staph aureus
Clinical Presentation
Rash / Skin Findings
- Expanding area of erythema that is warm and tender with poorly defined borders
- Skin may appear shiny and tight with localized swelling
- Unilateral, most commonly involving one shin or covering one lower leg
- No vesicles, no crusting
Systemic Symptoms
- ± fever, chills, malaise
The question stem will likely include
- An expanding, warm, tender erythematous area with poorly defined borders on one shin or covering one lower leg.
Physical Exam & Labs
- Diagnosis is clinical
- Area is warm, tender, and edematous
Treatment
- Mild / uncomplicated (oral): Cephalexin, dicloxacillin, or amoxicillin-clavulanate (β-lactams; streptococcal ± MSSA coverage)
- If MRSA risk factors: TMP-SMX, doxycycline, or clindamycin (MRSA-active antibiotics)
- Severe/systemic (IV): Cefazolin or ceftriaxone; vancomycin if MRSA concern (IV β-lactams ± glycopeptide)
Exam Keys
- Poorly defined borders = cellulitis
- Deeper infection than erysipelas
- Think oral vs IV, not topical
Erysipelas
- Acute bacterial infection of the superficial dermis and lymphatics
- Most commonly caused by Strep pyogenes
Clinical Presentation
Rash / Skin Findings
- Bright red, erythematous plaque with sharply demarcated, raised borders
- Skin may appear tense and indurated
- Commonly involves the face or lower extremities
- Usually unilateral
Systemic Symptoms
- Fever and chills are common
- Malaise may be present
The question stem will likely include
- A patient with fever and a raised, sharply demarcated erythematous lesion on the face (may also involve the lower extremities).
Physical Exam & Labs
- Diagnosis is clinical
- Lesion is warm, tender, and well demarcated
Treatment
- Uncomplicated (oral): Penicillin, amoxicillin, or cephalexin (β-lactams; streptococcal coverage)
- Severe/systemic (IV): Penicillin G or cefazolin (IV β-lactams)
Exam Keys
- Sharp, raised borders = erysipelas
- More superficial than cellulitis
- Strep coverage is sufficient
- Face involvement is a classic clue
Impetigo
- Superficial bacterial skin infection
- Most commonly caused by Strep pyogenes or Staph aureus
- Most common in children
Clinical Presentation
Rash / Skin Findings
- Vesicles or pustules that rupture to form honey-colored crusted lesions
- Typically involves the face, especially around the mouth and nose
- May spread rapidly; highly contagious
The question stem will likely include
- A child with honey-colored crusted lesions on the face, often around the mouth or nose
Physical Exam
- Diagnosis is clinical
- Lesions are superficial with characteristic crusting
Treatment
- Localized disease: Mupirocin or retapamulin (topical antibiotics)
- Extensive disease or outbreaks: Cephalexin or dicloxacillin (oral β-lactams; streptococcal ± MSSA coverage)
- If MRSA concern: Clindamycin (MRSA-active antibiotic)
Exam Keys
- Honey-colored crusts = impetigo
- Superficial infection
- Think topical first unless extensive
- Pediatric population is classic
Candidiasis (Cutaneous)
- Superficial fungal infection caused by Candida species
- Thrives in warm, moist environments
Clinical Presentation
Rash / Skin Findings
- Beefy red, erythematous rash
- Involves skin folds or diaper area
- Satellite pustules or papules at the periphery
- May be macerated or moist
The question stem will likely include
- A beefy red rash with satellite lesions in skin folds or an infant with a diaper rash not improving with barrier creams.
Physical Exam
- Diagnosis is clinical
- Moist erythematous plaques with satellite lesions
Treatment
- Topical azoles or nystatin (topical antifungals)
- Oral fluconazole if severe or refractory (systemic azole)
Exam Keys
- Satellite lesions = candidiasis
- Think moist areas
- Diaper rash + beefy red = Candida
- Topical first
Dermatophyte Infections (Tinea)
- Superficial fungal infections caused by dermatophytes
- Named by location (tinea corporis, pedis, cruris, capitis)
Clinical Presentation
Rash / Skin Findings
- Annular (ring-shaped), scaly plaques with central clearing
- Raised, scaly border
- Typically dry
- Pruritus common
The question stem will likely include
- An itchy, annular, scaly rash with central clearing.
Physical Exam
- Diagnosis is clinical
- Scaling most prominent at the edges
Treatment
- Topical azoles or allylamines (e.g., clotrimazole, terbinafine)
- Oral antifungals for extensive disease or tinea capitis
Exam Keys
- Central clearing = dermatophyte
- Dry, scaly > moist
- Wrestler = tinea corporis
- Topical first unless scalp or extensive
Scabies
- Parasitic skin infestation caused by Sarcoptes scabiei
- Spread by skin-to-skin contact
Clinical Presentation
Rash / Skin Findings
- Intensely pruritic papules or nodules
- Linear or burrow-like lesions
- Involves finger webs, wrists, axillae, waistline, genital area
- Pruritus is worse at night
The question stem will likely include
- Intense nocturnal itching with linear or burrow-like lesions in the finger webs or wrists.
Physical Exam
- Diagnosis is clinical
- Burrows or excoriated papules in classic locations
Treatment
- Permethrin cream (topical antiparasitic)
- Oral ivermectin for severe or refractory cases
- Treat all close contacts simultaneously
Exam Keys
- Nighttime itching = scabies
- Linear/burrow-like lesions are key
- Finger webs are classic
- Treat contacts even if asymptomatic
Lice (Pediculosis)
- Parasitic infestation caused by lice
- Most commonly pediculosis capitis (head lice)
Clinical Presentation
Rash / Skin Findings
- Scalp pruritus
- Nits (eggs) firmly attached to hair shafts
- Excoriations from scratching
- Common behind the ears and at the nape of the neck
The question stem will likely include
- Scalp itching with visible nits attached to hair shafts, especially behind the ears or at the neck.
Physical Exam
- Diagnosis is clinical
- Nits firmly adherent to hair (do not flake off)
Treatment
- Permethrin lotion (topical pediculicide)
- Pyrethrins or malathion as alternatives
- Mechanical removal of nits recommended
Exam Keys
- Nits stuck to hair = lice
- Scalp involvement
- Itching without burrows
- Treat household contacts as needed
Spider Bites
Brown Recluse
- Necrotic arachnidism
- Often associated with south-central U.S
- Early bite may be painless or mild, necrosis develops later
- Local tissue destruction
Clinical Presentation
- Initially mild bite → progressive pain
- Central necrosis with surrounding erythema
- Possible eschar formation
The question stem will likely include
- A painful bite that develops central necrosis over hours to days.
Management
- Local wound care
- Pain control
- No routine antivenom
Exam Keys
- Necrosis = brown recluse
- Local tissue injury, not muscle cramps
Black Widow
- Neurotoxic envenomation
- Systemic symptoms predominate
Clinical Presentation
- Bite may be minimal
- Severe muscle cramps, abdominal pain
- Diaphoresis, hypertension possible
The question stem will likely include
- Severe muscle cramping or abdominal pain after a spider bite.
Management
- Supportive care
- Muscle relaxants, analgesia
- Antivenom if severe
Exam Keys
- Muscle cramps = black widow
- Systemic > local findings
Tick Bites
Big exam idea:
The exam is testing whether this is a local bite reaction or early Lyme disease.
Lyme disease more common in Northeast and Upper Midwest
Local Tick Bite Reaction
Clinical Presentation
- Small erythematous papule at bite site
- Appears soon after tick removal
- Does not expand
- Resolves over 1–2 days
The question stem will likely include
- A small, localized red lesion shortly after a tick bite that does not expand.
Management
- Reassurance
- Symptomatic care only
Exam Keys
- Localized and improving = not Lyme
- No antibiotics required
Erythema Migrans (Early Lyme Disease)
- Characteristic rash of early Lyme disease
- Indicates systemic infection, not a local reaction
Clinical Presentation
- Expanding erythematous rash, usually >5 cm
- Develops days to weeks after tick bite
- May have central clearing but bull’s-eye is not required
- Usually not painful or pruritic
The question stem will likely include
- An expanding erythematous rash days to weeks after a tick bite.
Management
- Treat clinically without waiting for labs
- Doxycycline is first-line (amoxicillin if pregnant or child)
Exam Keys
- Expanding rash = erythema migrans
- Erythema migrans = Lyme disease
- No serology needed to start treatment
- Do not confuse with cellulitis or bite reaction
Bee / Wasp Stings
Clinical Presentation
- Local reaction: pain, erythema, swelling
- Systemic reaction: urticaria, wheeze, hypotension
The question stem will likely include
- Rapid onset of hives, wheezing, or hypotension after a sting.
Management
- Local care for mild reactions
- Epinephrine for anaphylaxis
Exam Keys
- Airway or hypotension = epinephrine
- Local swelling alone is not an allergy
Bedbugs / Fleas
Clinical Presentation
- Grouped or linear pruritic papules
- Often on exposed skin
- History of travel, bedding, or pets
The question stem will likely include
- Linear clusters of itchy bites appearing overnight.
Management
- Symptomatic treatment
- Environmental control
Exam Keys
- Linear pattern
- No systemic illness
Envenomation Exam Summary
- Necrosis → brown recluse
- Muscle cramps → black widow
- Expanding rash after tick bite → erythema migrans
- Hypotension/wheeze after sting → anaphylaxis
- Linear itchy bites overnight → bedbugs/fleas
