Psoriasis is a chronic inflammatory skin disease associated with numerous comorbidities, including increased risk of psoriatic arthritis (PsA) in up to 30% of patients and kidney damage, which may eventually progress to end-stage renal disease (ESRD). The risk of developing chronic kidney disease (CKD) and ESRD is elevated, with odds ratios of 1.65 (95% CI, 1.29–2.12) and 1.37 (95% CI, 1.14–1.64), respectively. The following case is an example of a patient with generalized plaque psoriasis who, over the years, developed kidney damage due to IgA nephropathy (IgAN). A 52-year-old female patient was admitted to the Clinic of Nephrology due to the exacerbation of IgAN, with a creatinine level of 2.8 mg/dL and an eGFR of 19 mL/min/1.73 m². The patient had suffered from chronic plaque psoriasis since the age of 15. At 38, she was diagnosed with sacroiliac joint involvement due to PsA. At that time, her creatinine level was 0.91 mg/dL. At 44, a decline in kidney function was noted (eGFR of 45.1 mL/min/1.73 m²), and a kidney biopsy revealed moderately advanced, focal, sclerosing mesangial proliferative IgA glomerulonephritis with minimal, fresh extracapillary necrotic components and interstitial fibrosis of the renal cortex. It was suspected that the worsening of kidney function was related to the coexisting psoriasis. Therefore, the primary goal was to optimize psoriasis treatment. For about 10 years, the patient was treated with methotrexate (MTX), which effectively controlled psoriasis, but her kidney function parameters continuously deteriorated. Between 2015 and 2023, a decline in eGFR from 45.1 to 37.1 mL/min/1.73 m² was observed. Due to the coexisting CKD, it was decided to discontinue MTX to avoid drug-related nephrotoxicity — at that time, her creatinine level was 1.52 mg/dl (eGFR 37.1 mL/min/1.73 m²). As an alternative, systemic retinoid therapy was initiated to treat the underlying disease. Unfortunately, within a month, the treatment proved ineffective. In line with the current psoriasis treatment protocol, after exhausting conventional methods, the patient was offered biological therapy in April 2023. In September 2023, after completing the proper qualification process, guselkumab, an antibody targeting IL-23 — a cytokine involved in the pathogenesis of psoriasis and other autoimmune diseases such as IgAN — was introduced. The therapy turned out to be highly effective in treating psoriatic skin lesions, but during the six-month follow-up, no improvement in kidney function was observed. Instead, further deterioration occurred, and the patient reached stage G4 of CKD (eGFR 19 mL/min/1.73 m²). This case demonstrates the need for early and regular monitoring of other organ functions in patients suffering from psoriasis. Therapy may result in effective skin clearing while failing to alleviate other extracutaneous psoriasis manifestations. When biological treatment is introduced, such as guselkumab, the deterioration of kidney function should be taken into account and prevented by strict monitoring.