HIV antibody testing is indicated for this patient with a severe flare of psoriatic arthritis. HIV infection can trigger the onset of or exacerbate preexisting psoriatic arthritis and psoriasis. Skin and joint symptoms tend to be severe. Explosive onset or severe flare-up of psoriatic arthritis should therefore raise suspicion for concomitant HIV infection. This patient had previously well-controlled psoriatic arthritis until a recent severe flare, as manifested by severe pain and swelling of multiple joints and psoriasis, and should therefore be tested for HIV infection.

Although viral infections other than HIV can trigger flares of psoriasis, infectious mononucleosis is characterized by sore throat, lymphadenopathy, and splenomegaly, none of which (except adenopathy) is seen in this patient. Therefore, heterophile antibody testing is not indicated.

HLA-B27 is associated with psoriatic arthritis but not with psoriasis; furthermore, it neither confirms the diagnosis nor explains the flare-up of psoriasis.

The initial clinical manifestation of early localized Lyme disease is erythema migrans, an erythematous skin lesion that is noted in 70% to 80% of patients with confirmed infection. Early disseminated Lyme disease develops several weeks after the initial infection. Patients frequently present with a febrile illness associated with myalgia, headache, fatigue, and lymphadenopathy. Although Lyme disease may trigger a flare of psoriasis, as most acute infections can, it cannot account for the patient's thrush.

Streptococcal pharyngitis is a common trigger of guttate psoriasis, especially in children. Guttate psoriasis can also be the first sign of a flare in previously stable chronic plaque psoriasis. Guttate psoriasis consists of many small raindrop-like papules and plaques on the trunk. This patient's psoriatic pattern does not suggest guttate psoriasis, and testing for streptococcal infection is not indicated.