Medication side effect (Montelukast): Montelukast, a leukotriene receptor antagonist used for asthma, has been reported to cause urinary retention as a rare side effect. Given the patient is on montelukast daily, this could be a contributing factor.

Benign Prostatic Hyperplasia (BPH): Although less common in a 37-year-old, BPH can cause urinary retention. The sudden onset might be due to a significant increase in prostate size or a change in the bladder's ability to empty.
Urethral stricture: This condition, which can be asymptomatic until it causes significant obstruction, might lead to sudden urinary retention if the stricture becomes severe enough.
Neurogenic bladder: Conditions affecting the nerves controlling the bladder can lead to urinary retention. This could be due to various causes, including diabetes, although the patient's age and lack of mentioned comorbidities make this less likely.

Spinal cord injury or compression: Even without a history of injury, spinal cord issues (e.g., herniated disk, spinal stenosis) can cause sudden urinary retention. Missing this diagnosis could lead to permanent neurological damage if not promptly addressed.
Cauda equina syndrome: A serious condition where the nerves in the spinal canal are compressed, leading to neurological symptoms including urinary retention. It requires immediate medical attention to prevent long-term damage.
Prostate cancer: Although rare in a 37-year-old, prostate cancer can cause urinary retention. Given the potential severity of this condition, it should not be missed.

Bladder neck contracture: A rare condition that could cause urinary retention, more commonly seen in patients with a history of prostate surgery or radiation.
Phimosis: A condition where the foreskin cannot be fully retracted over the glans penis, potentially causing urinary retention if severe.
Urethral stones: Stones in the urethra can cause obstruction leading to urinary retention, though this would be an unusual presentation without other symptoms like pain.