A 7-year-old intact male Labrador Retriever presents with a 3-week history of progressive pelvic limb weakness, fecal tenesmus, and perineal swelling. Rectal palpation reveals a large, doughy mass lateral to the rectum that deviates it medially. Urinalysis shows isosthenuria. Which of the following is the MOST likely cause of the isosthenuria in this patient?

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