A 62-year-old man with fair skin and a history of outdoor occupational work presents with a slowly growing, painless lesion on the tip of his nose. On examination, the lesion shows a pearly, translucent papule with a rolled waxy border, prominent overlying blood vessels, and a central depression with crusting. The structure marked **A** in the diagram is suspected. Which of the following statements about this lesion is most consistent with current understanding of its pathogenesis and clinical behavior?

Question

Accepted Answer

A. It arises from dysregulation of the Hedgehog signaling pathway, most commonly due to loss-of-function mutations in PTCH1, and very rarely metastasizes (<0.1%). ## Why option 1 (correct) is right

The structure marked **A** is basal cell carcinoma (BCC), which is characterized by a pearly rolled border with telangiectasia and central ulceration (rodent ulcer) — the classic presentation in this case. According to Bolognia Dermatology 5e and NCCN guidelines, BCC arises from dysregulation of the **Hedgehog signaling pathway**, with **loss-of-function mutations in PTCH1** occurring in >90% of sporadic cases. Critically, although BCC is locally destructive and the most common human malignancy worldwide (accounting for ~80% of non-melanoma skin cancers), it **very rarely metastasizes (<0.1%)**. This low metastatic potential is a defining feature that distinguishes BCC from more aggressive skin malignancies and guides clinical management toward tissue-sparing approaches.

## Why each distractor is wrong

- **Option 0**: While correctly identifies PTCH1 loss-of-function as the pathogenic mechanism, it falsely claims a 5–10% metastatic potential. BCC metastasizes in <0.1% of cases — this overestimation is a critical error that would lead to inappropriate aggressive systemic therapy.
- **Option 2**: Incorrectly attributes BCC pathogenesis to Wnt/β-catenin dysregulation and TP53 mutations alone (TP53 mutations do occur but are not the primary driver). Furthermore, it wrongly equates BCC metastatic risk to melanoma, which is fundamentally incorrect — melanoma has far higher metastatic potential.
- **Option 3**: Falsely implicates BRAF mutations and MAPK pathway activation, which are hallmarks of melanoma and some other skin cancers, not BCC. Additionally, it incorrectly ranks BCC as the second most common non-melanoma skin cancer; BCC is the most common.

**High-Yield:** BCC = Hedgehog pathway (PTCH1 loss) + extremely low metastatic risk (<0.1%) + most common skin cancer = tissue-sparing surgery is safe and appropriate.

[cite: Bolognia Dermatology 5e Ch 108; NCCN Guidelines BCC 2024]

Answer

B. It arises from dysregulation of the Hedgehog signaling pathway, most commonly due to loss-of-function mutations in PTCH1, and has a metastatic potential of approximately 5–10%

Answer

C. It arises from dysregulation of the Wnt/β-catenin signaling pathway due to TP53 mutations alone, and has a metastatic potential similar to cutaneous melanoma

Answer

D. It arises from constitutive activation of the MAPK pathway due to BRAF mutations, and is the second most common non-melanoma skin cancer worldwide

Explanation

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