The correct answer is D. Bullous erythema multiforme.
The combination of penile erosions and a targetoid tense vesicle on the base of this patient’s left thumb makes the diagnosis of bullous erythema multiforme most likely.
Primary syphilis often produces a painless genital chancre, but a concurrent acral targetoid tense vesicle makes this diagnosis less likely. Secondary syphilis often produces lesions on acral sites, but these lesions are papulo-squamous rather than vesico-bullous. Pemphigus vulgaris and bullous pemphigoid typically present with more widespread involvement than is pictured here.
References: PMID: 24819646