Benign Prostatic Hyperplasia (BPH): The patient's age and symptoms of trouble emptying the bladder are highly suggestive of BPH, a common condition in older men where the prostate gland enlarges, potentially obstructing the flow of urine.

Urinary Retention due to Medication Side Effects: Certain medications, such as anticholinergics, can cause urinary retention. Given the patient's age, it's possible they are on medications that could contribute to their symptoms.
Neurogenic Bladder: Conditions affecting the nerves that control bladder function, such as diabetes or stroke, could lead to urinary retention. The patient's age increases the likelihood of such underlying conditions.
Prostatitis: Inflammation of the prostate, which could be chronic, might cause urinary symptoms similar to BPH.

Prostate Cancer: Although the PSA is 1.1, which is within the normal range, prostate cancer must be considered, especially in older men. A digital rectal exam (DRE) and further evaluation might be necessary.
Spinal Cord Injury or Compression: Conditions like spinal stenosis or a herniated disk could compress nerves and lead to urinary retention. Missing this diagnosis could result in permanent neurological damage.
Bladder Outlet Obstruction due to Other Causes: Such as bladder stones or urethral stricture, which are less common but could have serious consequences if not addressed.

Bladder Dysfunction due to Parkinson's Disease or Multiple System Atrophy: These neurodegenerative disorders can affect bladder control, leading to urinary retention among other symptoms.
Sphincter Dyssynergia: A condition where the external urethral sphincter does not relax properly during urination, causing obstruction. This is more commonly seen in younger individuals with neurological disorders but should be considered in the differential diagnosis of urinary retention.