Causes of urinary retention
These are numerous and can be classified as:

In men – benign prostatic hypertrophy (BPH), meatal stenosis, paraphimosis, penile constricting bands, phimosis, prostate cancer.

In women – prolapse (cystocele, rectocele, uterine), pelvic mass (gynaecological malignancy, uterine fibroid, ovarian cyst), retroverted gravid uterus.

In both – bladder calculi, bladder cancer, faecal impaction, gastrointestinal or retroperitoneal malignancy, urethral strictures, foreign bodies, stones.

Infectious and inflammatory:

In men – balanitis, prostatitis and prostatic abscess.
In women – acute vulvovaginitis, vaginal lichen planus and lichen sclerosis, vaginal pemphigus.
In both – bilharzia, cystitis, herpes simplex virus (particularly primary infection), peri-urethral abscess, varicella-zoster virus.

Drug-related:
Up to 10% AUR episodes are thought to be attributable to drugs. Those known to increase risk include:

Anticholinergics (eg, antipsychotic drugs, antidepressant agents, anticholinergic respiratory agents).
Opioids and anaesthetics.
Alpha-adrenoceptor agonists.
Benzodiazepines.
Non-steroidal anti-inflammatory drugs.
Detrusor relaxants.
Calcium-channel blockers.
Antihistamines.
Alcohol.

Neurological:
More often causing chronic retention but may cause AUR( Acute Urinary retention).

Autonomic or peripheral nerve (eg, autonomic neuropathy, diabetes mellitus, Guillain-Barré syndrome, pernicious anaemia, poliomyelitis, radical pelvic surgery, spinal cord trauma, tabes dorsalis).
Brain (eg, cardiovascular disease (CVD), MS, neoplasm, normal pressure hydrocephalus, Parkinson’s disease).
Spinal cord (eg, invertebral disc disease, meningomyelocele, MS, spina bifida occulta, spinal cord haematoma or abscess, spinal cord trauma, spinal stenosis, spinovascular disease, transverse myelitis, tumours, cauda equina).

Other:

In men – penile trauma, fracture, or laceration.
In women – postpartum complications (increased risk with instrumental delivery, prolonged labour and Caesarean section),urethral sphincter dysfunction (Fowler’s syndrome).
In both – pelvic trauma, iatrogenic, psychogenic.

BPH-Benign Prostatic Hypertrophy- is by far the most common cause of urinary retention.

AUR is often encountered postoperatively and the reasons for this are multifactorial:

Pain.
Traumatic instrumentation.
Bladder overdistension.
Drugs (particularly opioids).
Iatrogenic (eg, suburethral sling procedures for stress incontinence[3], posterior colporrhaphy[4]).
Decreased mobility and increased bed rest.