What is the most likely diagnosis for a 1‑inch bald, rough, mildly erythematous patch on the right side of the scalp in a 27‑year‑old woman?

Differential Diagnosis: Bald, Rough, Erythematous Scalp Patch in a 27-Year-Old Woman

The most likely diagnosis is tinea capitis (scalp ringworm), and you must obtain fungal culture or KOH preparation to confirm before initiating systemic antifungal therapy. 1, 2

Primary Diagnostic Consideration

Tinea capitis should be your leading diagnosis given the combination of localized alopecia with a rough (scaling) texture and mild erythema in a discrete patch. 1, 2 While tinea capitis predominantly affects children aged 3-7 years, it occurs in adults—particularly postmenopausal women and those of African descent—and is frequently misdiagnosed or diagnosed late because clinicians maintain a low index of suspicion in this age group. 3, 4

Key Clinical Features Supporting Tinea Capitis

• The "rough patch" indicates scaling, which is characteristic of non-inflammatory tinea capitis that can present as fine scaling with circular alopecia (grey patches) or diffuse scaling resembling dandruff with subtle hair loss. 2

• Mild erythema with localized alopecia fits the non-inflammatory presentation, though inflammatory variants (kerion) would show more pronounced inflammation. 2, 5

• The scalp may appear "slightly inflamed" in tinea capitis, but signs can be subtle, which matches your patient's "slightly reddened" presentation. 1

Critical Differential Diagnoses to Exclude

The British Association of Dermatologists guidelines specifically list conditions that cause diagnostic confusion with alopecia areata, but several apply here: 1

Alopecia Areata

• Less likely because alopecia areata typically presents with smooth, non-scaly patches of hair loss, whereas your patient has a "rough patch" indicating scale. 1
• The affected skin in alopecia areata may be "slightly reddened but otherwise appears normal" without the rough/scaling texture described here. 1
• Dermoscopy would show yellow dots and exclamation mark hairs in alopecia areata, not present in tinea capitis. 1

Trichotillomania

• Shows incomplete hair loss with firmly anchored broken hairs (still in anagen phase), unlike the complete alopecia with scaling seen in fungal infections. 1

Early Scarring Alopecia

• Consider this if the patch shows follicular dropout on close examination, but the presence of scale and erythema makes an infectious etiology more likely initially. 1

Diagnostic Workup

Obtain fungal culture and/or KOH preparation immediately—these are the gold standard confirmatory tests. 2, 3

• Fungal culture is preferred as it identifies the specific dermatophyte (most commonly Trichophyton tonsurans or Microsporum canis in adults). 2, 3

• KOH preparation provides rapid results showing fungal elements. 2, 4

• Dermoscopy (trichoscopy) is highly sensitive and can show characteristic findings that distinguish tinea capitis from other causes of scalp alopecia. 2, 3

• Skin biopsy should be reserved for cases where the diagnosis remains uncertain after initial testing, particularly to exclude scarring alopecia or lupus. 1

Management Algorithm

If Tinea Capitis is Confirmed:

Initiate oral antifungal therapy immediately—topical agents alone are inadequate because they do not penetrate hair follicles deep within the dermis. 2, 3

First-line options for adults (extrapolated from pediatric guidelines as no adult-specific guidelines exist): 3

• Terbinafine: Preferred in adults who can swallow tablets; most cost-effective with robust efficacy data. 6, 3

• Griseofulvin: Traditional gold standard at 20-25 mg/kg/day for 8 weeks, though longer treatment duration is a disadvantage. 6, 2

• Fluconazole or itraconazole: Alternative options with shorter treatment courses (≤4 weeks). 6, 2

Adjunctive topical therapy with sporicidal shampoos (selenium sulfide) reduces spore transmission and aids scale removal, though not curative alone. 6, 2

If Diagnosis Remains Uncertain:

Proceed with fungal culture, KOH, and consider biopsy to exclude early scarring alopecia, lupus, or secondary syphilis if risk factors are present. 1

Critical Pitfalls to Avoid

• Do not treat empirically for seborrheic dermatitis or psoriasis without confirming the diagnosis—resistance to these treatments should immediately prompt fungal studies. 4

• Do not rely on topical antifungals alone if tinea capitis is confirmed, as treatment failure is inevitable. 2, 3

• Do not delay diagnosis—inadequate or delayed treatment can result in permanent scarring alopecia, particularly with inflammatory variants. 5, 3

• Obtain mycologic confirmation before starting systemic antifungals to ensure appropriate agent selection and avoid unnecessary treatment. 2