Vitiligo questions feel deceptively “easy” until the answer choices start looking the same: hypopigmented vs depigmented, autoimmune vs post-inflammatory, “wood’s lamp” vs KOH. This post breaks down a classic USMLE-style vignette and—more importantly—why every distractor is there.
Tag: Dermatology > Skin Disorders
Clinical Vignette (USMLE-Style)
A 24-year-old woman presents with “white patches” on her hands and around her mouth for 8 months. They have gradually expanded. She reports no itching or pain. Past history includes autoimmune thyroid disease. Physical exam shows well-demarcated, chalk-white macules and patches on the dorsal hands and perioral region. There is no scale. A Wood lamp exam makes the lesions appear bright blue-white.
Which of the following is the most likely diagnosis?
A. Tinea versicolor
B. Pityriasis alba
C. Vitiligo
D. Post-inflammatory hypopigmentation
E. Pityriasis rosea
Correct Answer: C. Vitiligo
Why it’s vitiligo
Key stem clues are doing heavy lifting:
- Chalk-white, well-demarcated macules/patches → suggests depigmentation (loss of melanin), not just “lighter skin”
- No scale → argues against many superficial fungal/inflammatory causes
- Autoimmune association (thyroid disease) → classic comorbidity
- Wood lamp: bright blue-white accentuation → supports true depigmentation
Pathophysiology (high yield)
Vitiligo is due to autoimmune destruction of melanocytes, leading to complete loss of melanin in affected skin.
- Often associated with other autoimmune diseases:
- Hashimoto thyroiditis / Graves
- Type 1 diabetes
- Pernicious anemia
- Addison disease
- Alopecia areata
Diagnosis pearls
- Clinical + Wood lamp is often enough.
- Biopsy (if needed): absence of melanocytes in epidermis.
Treatment (Step-friendly)
- Topical corticosteroids (limited disease)
- Topical calcineurin inhibitors (esp face/intertriginous)
- Narrowband UVB phototherapy (more extensive)
- Sun protection + cosmetic camouflage are supportive
Why Each Distractor Is Wrong (and What It Would Look Like)
A. Tinea versicolor (Malassezia)
Why it’s tempting: “Light patches” is a common student trap.
Why it’s wrong here:
- Usually has fine scale (may be subtle but present)
- Common on trunk/shoulders, less so perioral/hands
- Wood lamp can show yellow-green/coppery fluorescence, not bright blue-white
- KOH would show “spaghetti and meatballs” (hyphae + spores)
Classic vignette: Teen/young adult with hypopigmented or hyperpigmented patches on upper trunk that are more noticeable after tanning.
High-yield table:
| Feature | Vitiligo | Tinea versicolor |
|---|---|---|
| Pigment change | Depigmented (chalk-white) | Hypo- or hyperpigmented |
| Borders | Well-demarcated | Less sharply demarcated |
| Scale | Absent | Present (fine) |
| Wood lamp | Bright blue-white | Yellow-green/coppery |
| Test | Clinical ± biopsy | KOH: spaghetti & meatballs |
B. Pityriasis alba
Why it’s tempting: Hypopigmented patches, often on the face.
Why it’s wrong here:
- Pityriasis alba is typically:
- Children/atopic background
- Ill-defined hypopigmented patches
- Fine scale
- Not “chalk-white” and not strongly associated with autoimmune disease
Classic vignette: Child with a history of eczema who has faint, poorly demarcated hypopigmented patches on cheeks.
Key distinction:
- Vitiligo = depigmented + sharply demarcated
- Pityriasis alba = hypopigmented + fuzzy borders + mild scale
C. Vitiligo (Correct)
If you remember only one phrase: “Well-demarcated depigmented patches + autoimmune associations.”
Extra high-yield add-ons:
- Can show Koebner phenomenon (lesions at sites of trauma)
- Can involve hair: poliosis (white hair)
- Increased risk of sunburn in affected areas due to absent melanin
D. Post-inflammatory hypopigmentation
Why it’s tempting: Hypopigmentation after inflammation/trauma is common.
Why it’s wrong here:
- The stem gives no preceding rash, burn, procedure, or inflammatory dermatitis
- Post-inflammatory hypopigmentation is usually:
- Less sharply demarcated
- Incomplete pigment loss (not chalk-white)
- Often matches distribution of prior dermatitis/lesion
Classic vignette: Patient with resolved eczema, psoriasis, or chemical irritation now has lighter areas where the rash used to be.
Board tip: If there’s no “before,” be skeptical of “post-inflammatory.”
E. Pityriasis rosea
Why it’s tempting: Students remember it as a common benign rash diagnosis and may over-pick it.
Why it’s wrong here:
- Pityriasis rosea is not primarily a “white patch” disease.
- Typical findings:
- Herald patch followed by multiple salmon-colored oval lesions
- Collarette scale
- “Christmas tree” distribution on trunk
- Not usually on hands/perioral region and not depigmented.
Classic vignette: Teen/young adult with a single patch then widespread trunk eruption after a mild URI-like prodrome.
High-Yield “Depigmented vs Hypopigmented” Cheat Sheet
Depigmented (think: melanocytes gone)
- Vitiligo (autoimmune melanocyte destruction)
- Chemical leukoderma (exposure-related)
Hypopigmented (think: melanin decreased, melanocytes present)
- Tinea versicolor (fungal effect on pigmentation)
- Pityriasis alba (mild eczema-related)
- Post-inflammatory hypopigmentation
- Nevus depigmentosus (congenital, stable)
Quick Exam Strategy: How to Nail Vitiligo in 10 Seconds
Look for:
- Chalk-white, well-demarcated patches
- No scale
- Autoimmune comorbidities
- Wood lamp bright blue-white accentuation
If the vignette mentions scale or trunk predominance, pivot toward tinea versicolor or inflammatory dermatoses.
Take-Home Points (What USMLE Wants You to Learn)
- Vitiligo = autoimmune melanocyte destruction → depigmented, sharply demarcated patches
- Strong associations with thyroid disease and other autoimmune disorders
- Wood lamp helps distinguish true depigmentation from subtle hypopigmentation
- Many distractors hinge on scale, border definition, and distribution