A 45-year-old man with a 2-month history of painful oral erosions and recurrent blisters on the lower abdomen and inner thighs presents to dermatology. Histopathology shows intraepidermal acantholysis with suprabasal cleft formation and "tombstone" appearance. Immunofluorescence reveals IgG and C3 deposition in an intercellular pattern. The patient is seronegative for anti-desmoglein 3 (Dsg3) but positive for anti-desmoglein 1 (Dsg1) antibodies. Which of the following is the most likely diagnosis?

Explanation

## Diagnosis: Pemphigus Foliaceus ### Clinical Presentation The patient presents with: - Recurrent blisters on the lower abdomen and inner thighs - Painful oral erosions (atypical for classic PF, but see serology below) - 2-month progressive course ### Serological Profile — The Critical Differentiator | Feature | Pemphigus Vulgaris | Pemphigus Foliaceus | |---------|-------------------|---------------------| | **Anti-Dsg3** | **Positive** (required) | Negative | | **Anti-Dsg1** | Variable (±) | **Positive** | | **Oral involvement** | Yes (Dsg3 expressed in mucosa) | Typically absent | | **Level of acantholysis** | Suprabasal | Subcorneal/granular layer | **Key Point:** The question explicitly states the patient is **seronegative for anti-Dsg3 and positive for anti-Dsg1**. This serological profile is the hallmark of **Pemphigus Foliaceus (PF)**. Anti-Dsg3 antibodies are the defining feature of Pemphigus Vulgaris — their absence rules out PV by the desmoglein compensation theory (Mahoney et al., NEJM 1999; Amagai et al.). ### Why Not Pemphigus Vulgaris? Pemphigus Vulgaris requires anti-Dsg3 antibodies. Dsg3 is the dominant adhesion molecule in the suprabasal epidermis and mucous membranes. Without anti-Dsg3, the diagnosis of PV cannot be made. The explanation that "serology may lag" or represent a "Dsg3-seronegative variant" is not supported by standard textbook teaching (Fitzpatrick's Dermatology, 9e; Rook's Textbook of Dermatology). Seronegative PV is not a recognized diagnostic category in standard references. ### Histopathology Note While classic PF shows **subcorneal** acantholysis, early or atypical PF can occasionally show deeper clefting. The serological profile (Dsg3−/Dsg1+) is the gold-standard differentiator between PV and PF per current consensus guidelines. ### Why Not Other Options? - **IgA Pemphigus:** IgA (not IgG) deposition on immunofluorescence; Dsg1/Dsg3 negative - **Paraneoplastic Pemphigus:** Associated with neoplasm; antibodies against plakin family proteins (envoplakin, periplakin); polymorphous lesions - **Pemphigus Vulgaris:** Requires anti-Dsg3 positivity — absent here **High-Yield (Fitzpatrick's / Robbins):** The desmoglein compensation theory explains that Dsg1 and Dsg3 overlap in expression. In PF (Dsg1+ only), oral mucosa is spared because Dsg3 compensates. However, the serological signature Dsg3−/Dsg1+ = **Pemphigus Foliaceus** by definition. [cite: Fitzpatrick's Dermatology 9e, Ch 54; Robbins 10e Ch 25; Amagai M, J Invest Dermatol 1994]