What could cause suddenly urinary retention in a 16 year old female? could constipation or uti?

Medication reaction : Acute urinary retention in a teenager is most often a drug effect. Constipation or UTI would more likely cause urgency, frequency or dribbling. Inability to urinate at all should be dealt with immediately and handled by a qualified specialist.

The following:: Could be from drugs, severe costipation, and rarely: bladder stone, large ureterocele, imerforate hymen with compressing menstruation. Worth consulting pediatric urologist. Uti also unlikey unless retention was preceded by symptoms of burning on urination.

Retention : Sudden onset at age 16 suggests a new or transient problemas opposed to a congenital issues. I would recommend you see a community general urologist for further assessment.