Prioritize history questions that change the differential.
Clinical Cases · Think before the reveal
Work the case. Defend the next step.
Turn dermatology facts into clinic-ready reasoning—one decision at a time, from the opening history through follow-up.
Your role: recognize, describe, prioritize, explain, and escalate under supervision. These cases do not authorize independent diagnosis, prescribing, procedures, discharge decisions, or emergency management.
PurposePractice sequential clinical reasoning.
LevelsFoundation through resident refresher.
PrerequisitesMorphology, distribution, and focused differential.
Time5, 15, or 30 minutes per case.
Choose your depth
Every pathway reaches a useful decision.
Enter at your available time. Deeper paths add interpretation, management reasoning, and follow-up.
Quick · 5 minutesOne discriminating clue
Describe the morphology, choose a focused differential, and identify the clue that changes your leader.
Core · 15 minutesComplete one caseWork through history, exam, testing, diagnosis, communication, and follow-up.
Deep Dive · 30 minutesCompare three patternsComplete inflammatory, infectious, and urgent-triage reasoning, then calibrate your confidence.
Use location, number, lesion, color, configuration, surface, and distribution.
Rank the most likely, an important alternative, and a dangerous mimic when relevant.
Give a concise presentation and supervised next-step recommendation.
Case discovery
Pick the skill—not just the diagnosis.
A focused launch library avoids empty categories and zebra-heavy lists.
Infectious · Rotation15 min
The expanding annular plaque
Describe an annular scaly eruption, separate common mimics, and decide what question a bedside test should answer.
- Morphology before diagnosis
- Focused differential
- KOH interpretation
Inflammatory · Foundation8 min
Chronic extensor plaques
Use distribution, scale, nail examination, and symptoms to strengthen or weaken a psoriasis pattern.
- What else to examine
- No-test reasoning
- Severity and context
Medication eruption · Intern6 min
Rash plus mucosal pain
Recognize the history and examination features that should shift the goal from naming the rash to urgent escalation.
- Red flags
- Medication timeline
- Role-appropriate escalation
Featured sequential case · Rotation level
The expanding annular plaque
Complete each decision to unlock the next. Your progress stays in this browser session only.
Step 1 of 7
1 · Presenting complaint
A 24-year-old presents with a slowly enlarging, mildly pruritic plaque on the forearm.
2 · Focused history and examination
The lesion began after wrestling practice and has expanded despite several days of an over-the-counter topical corticosteroid. No fever, mucosal symptoms, or systemic illness is reported.
Examination: one well-demarcated annular plaque with an erythematous, scaly advancing edge and relative central clearing on the forearm.
3 · Build a focused differential
Prioritize common, important, and separating features.
4 · Choose and interpret a test
Ask first: what question are you trying to answer?
5 · What changes your mind?
KOH result: branching, septate hyphae are seen in material from the active edge.
6 · Diagnosis and management reasoning
Most likely diagnosis: tinea corporis.
Why it fits
Expanding annular plaque, active scaly border, exposure context, corticosteroid-associated alteration, and concordant KOH findings.
Why not the alternatives?
Nummular dermatitis is commonly coin-shaped but the advancing border and hyphae argue away from it. Annular psoriasis can scale, but the exposure, border activity, and fungal microscopy fit less well.
7 · Follow-up and teaching points
Finish the reasoning loop.
At follow-up, reassess clinical response, adherence, ongoing exposure or reinfection, extent, and whether the diagnosis should be reopened if the course is discordant. Persistent or extensive disease, hair or nail involvement, diagnostic uncertainty, or treatment complications may change evaluation and management.
An annular configuration narrows the differential; it does not name the diagnosis.
“Solitary annular erythematous plaque with an active scaly border and relative central clearing.”
Define the testing question, sample the appropriate area under supervision, and correlate the result with the clinical picture.
What would you say?
“This is a solitary, expanding annular plaque with a scaly active border and relative central clearing after a relevant exposure. Tinea corporis leads over nummular dermatitis and annular psoriasis. KOH from the active edge shows branching septate hyphae, supporting a clinically concordant dermatophyte infection. I would discuss site-appropriate therapy and follow-up with my supervisor.”
Clinical prioritization
Build a differential that earns every item.
Do not reward length. Reward fit, consequence, and discrimination.
What diagnosis often produces this morphology in this setting?
Which alternative would meaningfully change testing or management?
Which mimic cannot be safely missed when red flags are present?
Which less-common diagnosis belongs because a specific clue supports it?
What changes your mind?
Select one new finding. Then decide whether your goal remains outpatient diagnosis or shifts to urgent escalation.
Common mistakes
Correct the reasoning process—not just the answer.
Diagnosing before describing
Use morphology and distribution to create the problem representation first.
Anchoring on one clue
Ask whether the rest of the history and examination agree.
Ordering without a question
Name what uncertainty the test should resolve and how the result would change action.
Ignoring medications
Build a timeline that includes starts, stops, dose changes, and nonprescription agents.
Overstating certainty
Separate the leading diagnosis from what is proven, supported, or still uncertain.
Stopping at diagnosis
Plan response assessment, monitoring, exposure control, and when to reopen the differential.
If your attending asks
Answer with a decision and a reason.
What else would you examine?
Choose based on the case: scalp, nails, mucosa, palms and soles, genital skin, lymph nodes, or additional body sites. Explain how that finding would change the differential or urgency.
What is your focused differential?
Give the leader, one important alternative, and a dangerous mimic when relevant. State the feature that separates them.
Why this test?
State the clinical uncertainty, the information the test provides, and how a positive or negative result would alter reasoning.
When would you escalate?
Escalate promptly for toxic appearance, mucosal involvement, rapidly progressive disease, purpura with systemic symptoms, extensive blistering or detachment, suspected invasive infection, or another urgent feature—within your role and local workflow.
Rapid-fire retrieval
Try one question.
30-second recap
- Ask only questions that can change the differential, urgency, or next step.
- Describe before diagnosing.
- Rank a focused differential and name the discriminating feature.
- Define the diagnostic question before choosing a test.
- Reassess when new evidence conflicts with the original story.
- Finish with management reasoning, follow-up, and role-appropriate escalation.
Next best actions
Keep the case connected to the curriculum.
Also connects to Treatments, Procedures, Dermpath & Dermoscopy, Rotation Ready, and the Glossary.
Sources & page information
- Centers for Disease Control and Prevention. About Ringworm.
- Centers for Disease Control and Prevention. Clinical Overview of Ringworm.
- DermNet. Terminology in dermatology.
- American Academy of Dermatology. Psoriasis clinical guideline resources.
- U.S. Food and Drug Administration. Serious skin-reaction safety communication illustrating medication-associated red flags.