The correct answer is koebnerization of pre-existing psoriasis (choice “c”).
Tattoos have been known to cause bacterial infection (choice “a”), but this was unlikely given the diffuse nature of the rash and the lack of pain or adenopathy. Allergic reactions to tattoo dyes (choice “b”) are certainly common, but usually red or yellow dyes—which were not used for this tattoo—provoke the worst reactions. Furthermore, itching would have been a more prominent feature of the patient's complaint. Had it been fungal infection (choice “d”), the steroid cream would have made it worse.
One possibility remained: the so-called isomorphic phenomenon (otherwise known as koebnerization). First described by Heinrich Koebner in the mid-19th century, koebnerization is characterized by the appearance of psoriasis in traumatized skin such as surgical wounds, abrasions, burns, or even tattoos. Several other conditions also exhibit this same linear response to trauma, including warts, molluscum, and lichen planus.
To test for this diagnosis, corroborative findings of psoriasis were sought and found in the patient’s nails. His history of rashes on the knees and elbows also contributed to establishing the diagnosis. Moreover, his complaint of arthritis was quite suggestive of psoriatic arthropathy, which afflicts about 25% of patients with psoriasis and has little to do with the severity of the skin disease itself. Once the diagnosis became more apparent, the patient recalled a family history of psoriasis. Had any question remained, a biopsy could remove doubt.
For the patient, twice-daily application of a stronger steroid cream (augmented betamethasone) was prescribed. Though this quickly cleared the koebnerizing psoriasis, it is likely we haven’t seen the last of this disease.