Nummular Dermatitis Following an Insect Bite in a Patient With Activated Phosphoinositide-3-Kinase Delta Syndrome (APDS): A Case Report and Literature Review.

Activated phosphoinositide-3-kinase delta syndrome (APDS) is a rare primary immunodeficiency caused by gain-of-function mutations in the PIK3CD gene, resulting in the hyperactivation of the PI3K-Akt-mTOR pathway and subsequent immune dysregulation. Although recurrent sinopulmonary infections are typical, cutaneous manifestations remain poorly characterized. Nummular dermatitis is an uncommon presentation in this context and may reflect the underlying inflammatory imbalance. A 17-year-old female patient with genetically confirmed APDS (heterozygous PIK3CD c.3061G>A; p.Glu1021Lys) presented with erythematous papules that evolved into tense bullae on the lower limbs following an insect bite. Over subsequent weeks, multiple well-defined, coin-shaped eczematous plaques developed on the extremities and trunk, consistent with nummular dermatitis. The patient had a history of recurrent respiratory infections, chronic sinusitis, and otitis media, managed with intravenous immunoglobulin and prophylactic antibiotics. Topical betamethasone with fusidic acid and oral antihistamines produced partial improvement, though intermittent relapses occurred during follow-up. No systemic symptoms or new infectious complications were identified. This case represents an unusual dermatologic manifestation of APDS, where a minor external trigger led to persistent eczema-like lesions. Dysregulated PI3K-Akt-mTOR signaling may amplify local inflammation and impair epidermal barrier recovery. Recognition of atypical or chronic dermatitis in immunodeficient patients is crucial for early diagnosis, targeted treatment, and multidisciplinary management. Reporting such cases expands the clinical spectrum of APDS and underscores the importance of considering immune dysregulation in persistent cutaneous disorders.