Yellowish nail discoloration has a wide range of differential diagnoses. It may be associated with infectious or non-infectious etiologies, but discoloration can also result from the consumption or topical application of natural or synthetic compounds found in additives. This case report demonstrates that, in a patient with psoriasis, yellowish nail discoloration can be caused by over-the-counter vitamin C-containing skincare products, in addition to psoriasis exacerbation. A 24-year-old woman presented with yellow-orange discoloration of the fingernails that began two months prior and worsened over the last month. The discoloration was more prominent on the right hand and fluctuated, appearing lighter at night. Physical examination revealed yellow-orange discoloration extending from the distal nail plate to the perionychium of digits I-IV on the right hand and digits II-III on the left hand. Neither onychodystrophy nor nail pitting was observed. The patient had a history of psoriasis and psoriatic arthritis, which were re-evaluated due to recurring joint pain. After five weeks of treatment with 5 mg methotrexate, the discoloration did not fully resolve. A review of her skincare routine revealed that she had been using a new over-thecounter vitamin C-containing facial cream for six months. She applied the cream using her fingertips, predominantly the second to fourth fingers of the right hand. One week after discontinuing the vitamin C cream, the discoloration disappeared, with no onychodystrophy noted, as confirmed by dermoscopy performed by a dermatologist. Nail discoloration due to skincare product application should always be considered a potential benign cause. Recognizing such conditions requires a thorough review of the skincare routine, especially by
dermatologists, to identify possible active ingredients or vehicles responsible for the discoloration.