The correct answer is D. Reactive singlet oxygen.

The lesions seen here is an example of Bowen’s disease (superficial squamous cell carcinoma). Photodynamic therapy is a photochemical reaction in which a photosensitizing molecule under an activating wavelength reacts with oxygen in the tissue to create reactive singlet oxygen species. This leads to local necrosis.

The other options do not explain how photodynamic therapy works.

References:

Lui H, Szeimies RM. Photodynamic Therapy. In: Bologna J, ed. Dermatology. Philadelphia: Elsevier Saunders; 2012: 2237-2250.

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