The purpose of this clinical trial is to investigate whether adding high-dose vitamin D (2,400 IU daily) to standard medical treatment (solifenacin combined with behavioral therapy) is more effective than standard treatment alone for children with overactive bladder-wet (OAB-wet).

OAB-wet causes sudden urinary urges and frequent daytime or incontinence, which significantly impacts a child's quality of life and increases the family's caregiving burden. While solifenacin is a standard medication used to calm the bladder, many children do not achieve complete dryness. This study introduces the "Treat-to-Target" (T2T) approach, where clinicians and families set personalized "functional goals" (such as zero leakage) and monitor progress closely to adjust care.

The study aims to answer the following questions:

Does adding vitamin D help more children achieve their goal of "zero leakage" compared to standard treatment? Does vitamin D help repair bladder-related nerves, as measured by a specific marker in the urine? Does this combined approach reduce the family's expenses (like laundry costs and diaper use) and improve the child's self-esteem?

Participants will be randomly assigned to one of two groups for 12 weeks:

Intervention Group: Standard care (solifenacin + behavioral therapy) plus daily vitamin D (2,400 IU).

Control Group: Standard care (solifenacin + behavioral therapy) alone. Researchers will evaluate symptoms, vitamin D levels, and nerve repair markers at 6 and 12 weeks to determine the best treatment strategy for these children.

This randomized controlled trial utilizes a "Treat-to-Target" (T2T) framework to evaluate the incremental value of high-dose vitamin D supplementation as an add-on therapy to standard anticholinergic treatment (solifenacin) for pediatric idiopathic overactive bladder-wet (OAB-wet).

This study shifts the focus from simple symptom reduction to achieving predefined, patient-centered functional goals. While solifenacin targets the muscarinic receptors to inhibit detrusor overactivity (symptomatic relief), vitamin D is hypothesized to act as a neuromodulator that promotes neural repair-specifically the remyelination of pelvic nerves involved in bladder control.The study will enroll children with OAB-wet and documented vitamin D insufficiency/deficiency. The intervention group receives a multimodal start consisting of solifenacin, standard behavioral therapy (SU), and 2,400 IU/day of vitamin D3. The control group receives standard-of-care (solifenacin + SU).Key components of the study include:Goal Setting: At Week 0, clinicians and caregivers will establish a specific clinical target.Monitor and Adjust: At Week 6, serum 25(OH)D levels and voiding diaries will be reviewed to assess the trajectory toward the clinical target and the biochemical threshold of 40 ng/mL.Mechanism Validation: Urinary Myelin Basic Protein (uMBP) corrected by creatinine will be measured at baseline and Week6&12 to provide evidence of structural neural repair.Value-Based Assessment: A health economic evaluation will compare the total societal cost (medication costs vs. reduced caregiver burden and laundry expenses) between the two strategies.

By comparing these two pathways, the study aims to demonstrate that a multimodal T2T strategy incorporating vitamin D offers superior clinical achievement rates and better long-term functional outcomes than conventional pharmacological approaches.

Inclusion Criteria:

• Children older than or equal to 5 years of age with a diagnosis of wet OAB (the diagnosis followed the latest guidelines of ICCS) attending the outpatient clinic of the Department of Urology of the Affiliated Children's Hospital of Chongqing Medical University,
• children with serum vitamin D levels below 35 ng/ml as indicated by the tests conducted by the hospital
• children whose guardians have given their informed consent, are able to ensure compliance and have signed a paper-based informed consent form.

Exclusion Criteria:

• Those with urinary malformations or serious diseases (e.g., hypospadias, cryptorchidism, posterior urethral valvulae, vesicoureteral reflux, neurogenic bladder, urinary tumors, urinary stones, bladder and urethral injuries, etc.)
• Those with neurological disorders (e.g., epilepsy, spinal cord injuries, spinal dysplasia, spinal embolism syndrome, multiple sclerosis, and autism spectrum disorders, etc.)
• People with serious heart disease, abnormal liver and kidney function, lung disease, bone deformity, serious digestive tract disease, genetic metabolic disease
• People with history of gastrointestinal surgery and urological surgery
• People with dry stools and long-term constipation
• People who are taking anticonvulsant and antiepileptic drugs, hormones, and anti-tuberculosis drugs
• People with history of hypercalcemia, hyperphosphatemia with renal rickets
• People who have had unexplained hematuria and hematuria with renal rickets within the last year
• People who have been suffering from severe heart disease or chronic diabetes.
• participation in other clinical studies at the time of consultation or during the follow-up period of other clinical studies
• unwillingness to participate in this study