A 28-year-old woman presents with sudden onset of hair loss over the scalp for the past 3 weeks. On examination, she has multiple round, well-demarcated patches of complete alopecia on the vertex and temporal regions. The scalp appears smooth without scaling or erythema. Pull test is positive at the margins of the lesions. She denies any systemic symptoms, fever, or recent infections. Her past medical history is significant for vitiligo diagnosed 5 years ago. Laboratory investigations including CBC, thyroid function tests, and serology for syphilis are all normal. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Alopecia Areata ### Key Diagnostic Features **Key Point:** Alopecia areata is a non-scarring, autoimmune alopecia characterized by sudden onset of circumscribed hair loss with a positive pull test at the margins. ### Clinical Presentation This patient presents with the classic triad of alopecia areata: 1. **Sudden onset** — hair loss appeared over 3 weeks 2. **Round, well-demarcated patches** — vertex and temporal regions 3. **Smooth scalp without inflammation** — no scaling, erythema, or permanent scarring ### Examination Findings **High-Yield:** The positive pull test at the *margins* of lesions is pathognomonic. Hairs are easily extracted without pain, revealing short, tapered, pigmented roots ("exclamation mark" hairs on dermoscopy). ### Association with Autoimmune Conditions The patient's history of vitiligo is a significant clue — alopecia areata is associated with other autoimmune conditions: - Vitiligo (as in this case) - Thyroid disease (Hashimoto's, Graves') - Pernicious anemia - Systemic lupus erythematosus **Clinical Pearl:** Patients with alopecia areata should be screened for thyroid dysfunction and other autoimmune diseases, though this patient's thyroid function is normal. ### Why Normal Labs Don't Exclude Alopecia Areata Normal CBC, TFTs, and syphilis serology do not rule out alopecia areata — the diagnosis is primarily clinical. Alopecia areata is an autoimmune condition mediated by T-cell infiltration of hair follicles, not a systemic infection or metabolic disorder. ### Pathophysiology Alopecia areata results from: - Aberrant T-cell response against hair follicle antigens - Collapse of immune privilege in the hair follicle - Lymphocytic infiltration of the bulb ("swarm of bees" appearance on histology) [cite:Griffiths Textbook of Dermatology Ch 75]