The patient is exhibiting signs of urinary obstruction, which could be caused by benign prostatic hypertrophy, prostate cancer, or other reasons. Although diagnostic procedures are important, it is more important to determine the extent of damage to the upper urinary tract. Long term blockage of the lower urinary tract, as evidenced by urinary retention, can lead to hydronephrosis and permanent kidney damage. Digital rectal exam and prostate specific antigen blood test can be performed once the extent of the kidney damage is known and treated. A CT scan requires contrast; it is possible the patient has kidney damage; the contrast would worsen this damage and should be deferred until after kidney function is determined.