A 42-year-old woman with Type 1 diabetes mellitus (HbA1c 7.8%) presents with symmetric, painless plaques on the pretibial surfaces of both legs that began as red-brown papules 3 years ago. Examination reveals the lesions marked **A** in the diagram: well-demarcated plaques with a waxy, atrophic, yellow-brown center, prominent telangiectasias, and a slightly raised violaceous-erythematous border. Two older lesions show superficial ulcerations after minor trauma. A punch biopsy from the active inflammatory border is performed. Which of the following histopathologic findings is MOST characteristic of the lesion marked **A** and would distinguish it from granuloma annulare?

Question

Accepted Answer

C. Horizontally layered zones of necrobiosis alternating with bands of palisading granulomatous inflammation extending through the full dermis and into subcutaneous fat. ## Why option 1 is correct

The histopathologic hallmark of necrobiosis lipoidica (the lesion marked **A**) is the presence of horizontally layered zones of necrobiosis (degenerated collagen) alternating with bands of granulomatous inflammation composed of palisading histiocytes, lymphocytes, and multinucleated giant cells that extend through the full thickness of the dermis and into the subcutaneous fat. This distinctive pattern of full-thickness dermal involvement with alternating zones of necrobiosis and granulomatous inflammation is pathognomonic for NL and is the key feature that distinguishes it from other granulomatous dermatoses. (Bolognia Dermatology 5e Ch 93; Lepe StatPearls 2024)

## Why each distractor is wrong

- **Option 2**: This describes granuloma annulare, which presents with superficial perivascular granulomas, central mucin deposition, and minimal dermal involvement. The granulomas in GA are more superficial and lack the full-thickness dermal and subcutaneous involvement characteristic of NL.
- **Option 3**: This describes tuberculosis verrucosa cutis or cutaneous tuberculosis, which shows caseating granulomas with acid-fast bacilli. NL shows non-caseating granulomas without organisms on special stains, and the clinical presentation and associations differ entirely.
- **Option 4**: This describes simple dermal fibrosis or scar tissue, which lacks the active granulomatous inflammation and necrobiosis zones that define NL. This would not explain the active inflammatory border seen clinically.

**High-Yield:** Necrobiosis lipoidica = full-thickness dermal and subcutaneous granulomatous inflammation with alternating zones of necrobiosis; granuloma annulare = superficial perivascular granulomas with central mucin.

[cite: Bolognia Dermatology 5e Ch 93; Lepe StatPearls 2024; Reid Br J Dermatol 2013]

Answer

A. Dermal fibrosis with sparse lymphocytic infiltrate and preserved collagen architecture without necrobiosis

Answer

B. Superficial perivascular granulomas with central mucin deposition and minimal dermal involvement

Answer

D. Caseating granulomas with acid-fast bacilli and epithelioid histiocytes arranged in naked granulomas

Explanation

Why option 1 is correct

Why each distractor is wrong

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