A 65-year-old man has developed pruritus followed by blistering skin lesions over the trunk, legs, and arms over the past month. On physical examination, there are 1- to 4-cm tense bullae, particularly over flexural surfaces of the skin. A biopsy of one lesion is examined microscopically by direct immunofluorescence staining and shows a subepidermal bulla, with both IgG and C3 deposited linearly along the dermal-epidermal junction. He is treated with topical corticosteroids, and a month later the lesions are healed without scarring. Which of the following components of the skin has most likely been targeted by an autoantibody in this man?

Okay, let's tackle this question. The patient has blistering skin lesions, specifically tense bullae on flexural surfaces. The history of pruritus followed by blisters over a month is important. The biopsy shows a subepidermal bulla with IgG and C3 in a linear pattern along the dermal-epidermal junction. The treatment with topical corticosteroids worked, so it's probably a subacute or chronic condition that responds to steroids.

The key here is the immunofluorescence findings. Linear IgG and C3 along the basement membrane zone suggest an autoimmune blistering disease. The most common ones are pemphigoid diseases. Bullous pemphigoid (BP) is a classic one. BP is IgG autoantibodies against BP180 (type XVII collagen) and BP230. The subepidermal bulla and linear deposits are characteristic. The lesions are tense and heal without scarring, which is typical of BP. The treatment with topical steroids aligns with BP management.

Other possibilities like pemphigus vulgaris (suprabasal acantholysis, IgG in intercellular spaces) are less likely here. Linear IgG/C3 is more for pemphigoid. Linear IgG alone might be for epidermolysis bullosa acquisita, but C3 is also present here. So the target is likely BP180.

The options would include components like keratin, desmoglein (for pemphigus), BP180, or others. Since the correct answer is BP180, the distractors would be other antigens. So the answer is BP180.

**Core Concept**
This question tests the pathophysiology of autoimmune blistering diseases, specifically the role of autoantibodies targeting basement membrane zone proteins. The linear deposition of IgG and C3 along the dermal-epidermal junction is a hallmark of **bullous pemphigoid (BP)**, an autoimmune condition targeting hemidesmosomal components.

**Why the Correct Answer is Right**
The subepidermal bullae and linear IgG/C3 staining indicate **BP**, where autoantibodies target **BP180 (type XVII collagen)** or BP230. These proteins are critical for epidermal-dermal adhesion. The binding of IgG triggers complement activation (C3 deposition) and neutrophil recruitment, causing blister formation. The clinical features (tense bullae, flexural predilection, pruritus) and response to topical steroids align with BP. The absence of scarring confirms a subepidermal split, distinguishing it from other pemphigoid variants.

**Why Each Wrong Option is Incorrect**
**Option A:** *Desmoglein 3* is the target in pemphigus vulgaris, causing suprabasal acantholysis and flaccid bullae, not subepidermal blisters.
**Option B:** *Keratin intermediate filaments* are disrupted in epidermolysis bullosa simplex, a genetic disease with fragile skin and intraepidermal blisters.
**Option C:** *Laminin 5* is targeted in epidermolysis bullosa acquisita, which also causes subepidermal bullae

✓ Correct Answer: A. Hemidesmosome