Step 1 topics → Musculoskeletal, Skin, and Connective Tissue → Hidradenitis, Pyoderma Gangrenosum, Erythema Nodosum

Hidradenitis, Pyoderma Gangrenosum, Erythema Nodosum

USMLE Step 1 trap: Recommends debridement for pyoderma gangrenosum, ignoring pathergy and the need for immunosuppression. Pyoderma gangrenosum exhibits pathergy (worsening with trauma/debridement) and is treated with immunosuppression, not surgical debridement.

These three conditions — hidradenitis suppurativa (HS), pyoderma gangrenosum (PG), and erythema nodosum (EN) — are grouped together because they're all inflammatory skin/subcutaneous conditions with neutrophilic involvement and strong systemic associations. USMLE Step 1 tests them mostly through clinical vignettes where you have to recognize the condition from its distribution, lesion morphology, or associated disease. The tricky part is that all three can appear in patients with IBD, so the stem might make you think about the gut first and then ask about the skin finding. Don't get thrown off — treat each condition's features as its own distinct fingerprint.

The exam hits each one from a specific angle. For HS, it's about location (apocrine areas: axillae, groin, inframammary) and pathogenesis (follicular occlusion, not gland infection). For PG, the key trap is management — students see a necrotic ulcer and want to debride it, but that's exactly wrong because of pathergy. For EN, it's about locking in the pretibial location and recognizing it as septal panniculitis triggered by something systemic (sarcoidosis, strep, TB, OCP, IBD). USMLE Step 1 loves asking what to work up next after diagnosing EN.

What makes these tricky is that they look scarier than they are conceptually. Students over-medicalize HS by assuming it's eccrine gland disease (it's not — eccrine glands make sweat everywhere; apocrine glands are in the skin folds), and they misplace EN lesions onto the thighs or arms instead of the anterior shins. Nail the classic presentations and the one management pearl for PG, and you're covered for Step 1 purposes.

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Common misconceptions

Common mistake
Wrong: Pyoderma gangrenosum should be debrided aggressively like an infected wound.
Right: Pyoderma gangrenosum exhibits pathergy (worsening with trauma/debridement) and is treated with immunosuppression, not surgical debridement.
Pyoderma gangrenosum is not an infected wound — it's a neutrophilic dermatosis, meaning the destruction is immune-mediated, not microbial. Debridement triggers pathergy: the trauma of cutting worsens the ulceration and can cause catastrophic expansion of the lesion. The correct approach is immunosuppression (systemic steroids are first-line), and if a surgeon recommends aggressive debridement in a vignette, that's the wrong answer.
Common mistake
Wrong: Erythema nodosum lesions are found on the thighs or arms.
Right: Erythema nodosum classically presents as tender, raised, erythematous nodules on the anterior shins (pretibial), representing septal panniculitis.
Erythema nodosum has an almost pathognomonic location — the anterior shins, bilaterally. This is because septal panniculitis (inflammation of the fat lobule septa) preferentially occurs there due to hemodynamic factors in the pretibial subcutaneous tissue. If a vignette places tender red nodules on the thighs or arms, reconsider your diagnosis; if it says anterior shins in a young woman with recent strep throat or sarcoidosis, EN is your answer.
Common mistake
Wrong: Hidradenitis suppurativa involves eccrine (sweat) glands.
Right: Hidradenitis suppurativa involves apocrine gland-bearing areas (axillae, groin, inframammary folds) and is driven by follicular occlusion, not primary gland infection.
The name 'hidradenitis' sounds like it involves sweat (hidro-) glands, but HS is actually driven by follicular occlusion in apocrine gland-bearing skin, not primary eccrine gland disease. Eccrine glands are distributed all over the body; apocrine glands concentrate in the axillae, groin, and inframammary folds — exactly where HS lesions appear. The plugged follicle ruptures into the dermis, triggering the chronic inflammatory cycle, so this is fundamentally a follicular disease that secondarily involves the surrounding apocrine glands.
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What the exam tests

  1. Know the classic distribution of hidradenitis suppurativa (axillae, groin, inframammary folds), its risk factors (obesity, smoking, female sex), and first-line treatments (topical clindamycin, oral antibiotics, adalimumab for severe disease) — the exam will test whether you understand it as a follicular occlusion disease in apocrine-bearing areas, not a primary gland infection.
  2. Recognize pyoderma gangrenosum by its necrotic, violaceous-bordered ulcer, its systemic associations (IBD, RA, hematologic malignancy), and critically its pathergy — trauma or debridement worsens the wound, so treatment is immunosuppression (steroids, cyclosporine), not surgery.
  3. Identify erythema nodosum by its tender, raised, erythematous nodules on the anterior shins (pretibial), understand its histopathology (septal panniculitis — inflammation of subcutaneous fat septa), and know the triggers that should prompt workup (strep, sarcoidosis, TB, IBD, oral contraceptives, coccidioidomycosis).

Can you avoid these mistakes?

A 32-year-old obese woman presents with recurrent painful nodules, sinus tracts, and scarring in both axillae and the inguinal folds. What is the underlying pathogenesis, and why is this condition NOT simply an infection of sweat glands?
A patient with Crohn disease develops a rapidly expanding, painful ulcer with a necrotic center and violaceous, undermined borders on the leg. The surgical team wants to debride it. What do you tell them, and what treatment do you recommend instead?
A 28-year-old woman presents with bilateral tender, raised red nodules on her shins that appeared two weeks after a throat infection. Biopsy shows inflammation of the fibrous septa between fat lobules. Name the condition, the histopathologic pattern, and at least three other triggers you should rule out.
A USMLE Step 1 vignette describes a patient with ulcerative colitis who develops two separate skin findings: (1) painful recurrent abscesses in the groin and (2) a necrotic lower-leg ulcer that worsened after a minor cut. What are the two diagnoses, and what is the key management difference between them?

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