Benign Prostatic Hyperplasia (BPH): The patient's symptoms of nocturia and difficulty urinating are classic for BPH, a common condition in older men. The initial elevated PSA (8.2) that slightly decreased (7.1) could be due to BPH or other non-cancerous conditions affecting the prostate.

Prostatitis: This condition, which is an inflammation of the prostate, can cause urinary symptoms similar to BPH and may also elevate PSA levels.
Prostate Cancer: Although the slight decrease in PSA might suggest a non-cancerous cause, prostate cancer remains a possibility, especially given the patient's age and initial PSA level.

Urinary Retention due to Neurological Disorders: Conditions like multiple sclerosis, spinal cord injuries, or diabetic neuropathy can cause urinary retention and should not be missed due to their significant impact on quality of life and potential for serious complications.
Bladder Cancer: Although less likely than prostate issues given the symptoms, bladder cancer can cause similar urinary symptoms and is a critical diagnosis not to miss due to its potential for aggressive behavior.
Kidney Stones or Obstruction: These conditions can cause severe pain and urinary symptoms, including difficulty urinating, and require prompt diagnosis and treatment to prevent kidney damage.

Prostate Abscess: A rare but serious condition that can cause urinary symptoms, fever, and elevated PSA.
Sphincter of Oddi Dysfunction: A condition that affects the sphincter controlling the flow of bile and pancreatic juice into the intestine, which can sometimes cause urinary symptoms due to shared nerve roots, although this is quite rare and indirect.
Urethral Stricture: Narrowing of the urethra can cause difficulty urinating and is more common in men, but less likely than the other diagnoses listed here given the patient's age and presentation.