Review Question - QID 214705

A 9-year-old girl presents to the clinic with a 1-month history of voiding difficulties. The patient endorses urinary urgency, increased abdominal pressure, and does not report diurnal or nocturnal enuresis. She does not endorse fever, chills, night sweats, flank pain, dysuria, constipation, or diarrhea. Her mother reports multiple episodes in which she had to press on her daughter's abdomen to assist her daughter in voiding. She has not had any previous difficulty with micturition. The patient has a history of Down syndrome, is currently not on medication, and has no allergies. She had an ear infection that resolved with antibiotics 2 weeks prior. Her temperature is 98.6°F (37.0°C), blood pressure is 96/58 mmHg, pulse is 80/min, and respirations are 16/min. On physical exam, suprapubic swelling is noted. Kidney and urinary structures are unremarkable on ultrasonography. The postvoid residual (PVR) and uroflowmetry are normal. Laboratory results are shown below:

Urinalysis:
Specific gravity: 1.010
Glucose: Negative
Ketones: Negative
Blood: Negative
Leukocyte esterase: Negative
Nitrite: Negative
Protein: Negative

Urine microscopy:
WBC: 2/hpf
RBCL 0/hpf
Bacteria: None
Epithelial cells: None

What is the most likely cause of this patient's symptoms?