Which structure may swell and cause increased frequency of urination due to common medical problems in older men?

Prostate

Explanation:

    Benign prostate hyperplasia (BPH) is an enlargement (swell) of the gland caused by a benign proliferation of its cells. It mainly affects men over 40 years of age and may cause difficulty urinating, weakness and interruption of urine output, urinary incontinence, and increased frequency of urination.

   The most common symptom of BPH is the urge to urinate more often, including at night. This is because the prostate  presses the urethra, which conducts urine out of the body. Due to pressure, the bladder muscles work harder to release urine. The bladder may eventually begin to contract, even when only a small amount of urine is present, creating the urge to urinate more often.

all the above is what i would say