Dermatology Snapshots

Summary

This case report follows a 56-year-old man with a progressively worsening, intensely pruritic eruption initially presumed to be a drug reaction to tirzepatide. Despite discontinuation of the medication and multiple prednisone tapers, his eruption expanded. On dupilumab for months, the eruption evolved into nearly confluent papules with intermittent hemorrhagic crusting. A biopsy eventually showed a clonal CD30⁺ infiltrate, leading to a diagnosis of lymphomatoid papulosis (LyP). Methotrexate was initiated.

However, at the time the patient was scheduled to start phototherapy, clinicians reassessed him and noted widespread crusted plaques and weeping papules. A simple mineral oil preparation revealed numerous scabies mites, eggs, and feces—the true diagnosis. Retrospective review of the earlier biopsy even revealed a single mite leg that had been overlooked. Immunomodulatory therapies likely contributed to the eruption’s severity, including progression toward crusted scabies.

The case highlights how diagnostic error evolves when cognitive biases compound over time. Premature closure, anchoring, familiarity bias, and diagnostic momentum all played significant roles, delaying a simple bedside test that ultimately solved the case. The authors emphasize the importance of maintaining diagnostic humility, revisiting assumptions, and prioritising bedside tools—morphology, scrapings, biopsies—before escalating to targeted systemic therapies.

Cognitive Biases & Practical Tips

1. Premature Closure

Definition: Accepting a diagnosis before it is fully verified, stopping the search early.
How it showed up: The eruption was attributed to tirzepatide with no initial biopsy or scraping.
Practical Tips:

• Force yourself to document at least three differential diagnoses before committing.

• If treatment fails after a reasonable interval, restart the workup, not the prescription pad.

2. Diagnostic Momentum

Definition: Once a diagnosis is recorded, subsequent clinicians accept it unquestioningly.
How it showed up: The LyP label locked in further decisions—including starting methotrexate and planning phototherapy—despite clinical progression.
Practical Tips:

• At each follow-up, perform a fresh first-principles exam: pretend it’s a brand-new consultation.

• Before escalating therapy, ask: “Does the current morphology still fit the working diagnosis?”

3. Familiarity Bias

Definition: The tendency to diagnose conditions we commonly see or feel comfortable managing, even when the presentation is atypical.
How it showed up: The eruption was repeatedly interpreted through the lens of familiar entities—drug eruption, eczema, LyP—rather than less common but classic “great masqueraders” like scabies, especially its crusted variant.
Practical Tips:

• When a condition looks like something common but behaves unlike it (worsens despite correct therapy, spreads unexpectedly, or shows unusual morphology), pause and seek a disconfirming finding.

• Keep a short personal checklist of “conditions I often miss because they look like something else”—scabies, tinea incognito, CTCL, secondary syphilis.

4. Anchoring Bias

Definition: Fixating on the first piece of information or initial impression and failing to adjust when new data appears.
How it showed up: The case anchored on “drug eruption from tirzepatide.” Even as the timeline, distribution, and morphology became incompatible, clinicians continued to interpret each change as a variation of the initial idea.
Practical Tips:

• If the morphology outgrows or outpaces your initial hypothesis, force a structured re-evaluation: biopsy, scraping, repeat history.

• Use team-based diagnostics—ask a colleague: “What would you call this if you hadn’t read the notes?”