Manassas, Virginia: Police in the United States are taking a unique approach to collecting evidence in a ‘‘sexting’’ case involving a 17-year-old boy: Authorities want to take a photo of the teen’s erect penis to compare with a mobile phone video, his lawyers said.

The case in Northern Virginia has sparked anger from the boy’s family, local lawyers and legal observers, who say that the amount of time and resources spent by law enforcement on private messages between teens is excessive.

The teen’s lawyers are particularly incensed that investigators want to take him to a hospital for an injection that would force him to become erect.

The teen is facing two felony charges in juvenile court, manufacturing and distributing child pornography, which could lead not only to imprisonment until he’s 21 but also to inclusion on the state sex offender registry, at a judge’s discretion, for the rest of his life.

‘‘The prosecutor’s job is to seek justice,’’ said the teen’s defence lawyer, Jessica Harbeson Foster.