A 28-year-old woman presents with sudden onset of patchy hair loss over the scalp for the past 3 weeks. On examination, she has three well-demarcated, round areas of complete alopecia on the vertex and temporal regions. The scalp is otherwise normal without erythema or scaling. Pull test is positive in the affected areas, and dermoscopy reveals exclamation mark hairs at the margins. She denies any systemic symptoms. Her past medical history includes vitiligo diagnosed 5 years ago. What is the most likely diagnosis?

Explanation

## Diagnosis: Alopecia Areata ### Clinical Features Supporting the Diagnosis **Key Point:** Alopecia areata presents with sudden onset of well-demarcated, round or oval patches of complete hair loss without inflammation or scarring. ### Diagnostic Hallmarks | Feature | Finding in This Case | |---------|---------------------| | Onset | Sudden (3 weeks) | | Pattern | Well-demarcated, round patches | | Scalp appearance | Normal, non-scarring | | Pull test | Positive (easy hair extraction) | | Dermoscopy | Exclamation mark hairs (pathognomonic) | | Associated conditions | Vitiligo (autoimmune association) | **High-Yield:** Exclamation mark hairs are diagnostic — short, fractured hairs with a thickened, pigmented distal end and thin, depigmented proximal end, seen at the margins of active lesions. ### Pathophysiology 1. **Autoimmune mechanism**: T-cell mediated attack on hair follicle bulge stem cells 2. **Genetic predisposition**: HLA association (HLA-DQ3, HLA-DR4) 3. **Environmental trigger**: Stress, infection, or trauma may precipitate onset 4. **Association with other autoimmune conditions**: Vitiligo, thyroid disease, celiac disease — this patient's vitiligo history is a red flag **Clinical Pearl:** The positive pull test in alopecia areata reflects the ease with which dystrophic anagen hairs can be extracted due to loss of root sheath attachment. ### Why Alopecia Areata Is Most Likely - **Sudden onset** with **well-demarcated patches** rules out androgenetic alopecia (gradual, diffuse) - **Non-scarring** and **normal scalp** differentiate from scarring alopecias - **Positive pull test + exclamation mark hairs** are pathognomonic - **Vitiligo history** strongly suggests autoimmune etiology - **Trichotillomania** would show irregular patches with broken hairs of varying lengths, not the uniform exclamation mark morphology ```mermaid flowchart TD A[Patchy hair loss]:::outcome --> B{Onset pattern?}:::decision B -->|Sudden, round patches| C{Scarring?}:::decision B -->|Gradual, diffuse| D[Androgenetic alopecia]:::outcome C -->|No scarring| E{Pull test + dermoscopy?}:::decision C -->|Scarring present| F[Scarring alopecia]:::outcome E -->|Exclamation mark hairs| G[Alopecia areata]:::action E -->|Broken hairs, irregular| H[Trichotillomania]:::outcome ``` [cite:Fitzpatrick's Dermatology 9e Ch 77]