This patient should have a rapid plasma reagin test. He has scaling plaques on the trunk and proximal extremities after a recent upper respiratory tract infection, which is most typical of pityriasis rosea. The clinical appearance of scaling papules and plaques is similar to that of secondary syphilis, although pityriasis rosea typically spares the face, palms, and soles, whereas syphilis often affects the palms and soles. In a sexually active patient who is not consistently using protection and has palm involvement, secondary syphilis must be ruled out. The primary chancre is typically painless in syphilis and therefore may go unnoticed.

Hepatitis C virus testing is indicated for patients with lichen planus (LP), especially if oral. LP is an acute eruption of purple, pruritic, polygonal papules that most commonly presents on the flexural surfaces, especially the wrists and ankles. LP can also present in the mucous membranes (mouth, vaginal vault, and penis) with white plaques that, if uncontrolled, may ulcerate. This patient's physical findings are not consistent with LP.

Herpes simplex virus infection presents with grouped vesicles and erosions and no scaling plaques, and so polymerase chain reaction test is not necessary.

Because testing should be performed if there is any clinical concern for syphilis, not doing a work-up is not an acceptable treatment option.