Urinary Retention with Overflow Incontinence: This condition is characterized by the inability to fully empty the bladder, leading to straining during urination and a feeling of incomplete bladder emptying. The leakage of urine when moving or walking, known as overflow incontinence, occurs because the bladder is not fully emptied, causing it to spill over when the intra-abdominal pressure increases.

Overactive Bladder (OAB): Symptoms of OAB include urgency, frequency, and incontinence. While the primary complaint here is straining and feeling of incomplete emptying, some patients with OAB may experience urinary retention due to the bladder's inability to properly contract and empty.
Neurogenic Bladder: This condition results from a lack of bladder control due to a brain, spinal cord, or nerve problem. Symptoms can include urinary retention, incontinence, and recurrent infections. The long-standing nature of the symptoms without change could suggest a neurogenic cause.
Constipation: Chronic constipation can cause urinary retention and overflow incontinence by compressing the urethra or bladder neck, making it difficult to urinate.

Spinal Cord Injury or Tumor: Although less likely, any condition affecting the spinal cord can lead to neurogenic bladder symptoms, including urinary retention and incontinence. Early diagnosis is crucial for treatment and prevention of long-term damage.
Multiple Sclerosis: This autoimmune disease can affect the central nervous system, leading to neurogenic bladder symptoms among others. It's essential to consider this diagnosis due to its significant impact on the patient's quality of life and the availability of treatments.
Cauda Equina Syndrome: A serious condition that occurs when the nerves in the spinal canal are compressed, leading to loss of bladder and bowel control, among other symptoms. Prompt recognition and surgical intervention are critical to prevent permanent damage.

Bladder Outlet Obstruction: This could be due to various causes such as bladder stones, urethral stricture, or prostate issues (less common in a 16-year-old). Symptoms include difficulty starting urination, straining, and a feeling of incomplete bladder emptying.
Detrusor Underactivity: A condition where the bladder muscle does not contract properly, leading to difficulty emptying the bladder. It's less common and might be considered if other diagnoses are ruled out.