The correct answer is B. Adjuvant pembrolizumab. 

The correct answer is adjuvant pembrolizumab. This patient has a 2.3-mm ulcerated melanoma with negative sentinel lymph node biopsy, corresponding to stage IIB melanoma. Recent randomized clinical trials, including KEYNOTE-716, demonstrated that adjuvant PD-1 inhibition significantly improves recurrence-free and distant metastasis–free survival in patients with resected stage IIB/IIC melanoma.

The other answer choices are incorrect. Observation with routine skin checks was historically standard for stage II melanoma but is no longer preferred for stage IIB/IIC disease given the demonstrated benefit of adjuvant immunotherapy. Adjuvant dabrafenib plus trametinib is indicated for patients with BRAF-mutated stage III melanoma, not node-negative stage II disease. Adjuvant ipilimumab is associated with significantly higher toxicity and inferior tolerability compared to PD-1 inhibitors and is no longer preferred in the adjuvant setting. Radiotherapy to the nodal basin is not indicated in a patient with a negative sentinel lymph node biopsy and is generally reserved for select cases with nodal involvement or high-risk features after lymph node dissection.

References:

Hadfield M, Muzumdar S, Sullivan R …
Updates in adjuvant and neoadjuvant therapies for resectable cutaneous melanoma Journal of the American Academy of Dermatology, 2025; 94, 1206-1212