OAKLAND, Calif. — Dr. Kimberly Chang, a physician at a community clinic in Chinatown, will never forget the first young girl she suspected had been sold for sex.

Kalea, a 15-year-old Cambodian-American girl who grew up in Oakland, kept coming in to be examined for sexually transmitted diseases, the beginning of a grim cycle of diagnosis and treatment. “I started asking, ‘Are you having sex with new people?’ ” Dr. Chang, 37, recalled. “It was always, ‘No, no, no, no, no.’ Eventually she confided that she was worried about ‘a friend.’ That’s when I asked, ‘Are you trading sex for money?’ ”

Emerging from a long, dark tunnel, Kalea slowly began to spill her stories. How her father beat her. The childhood rape. The out-of-control john who tied her up in a motel bathtub and filled it with scalding water.

Seven years and hundreds of patients later, Dr. Chang’s clinic, Asian Health Services, is in the vanguard of a new public health approach to treating American-born minors lured into the sex trade, a problem enforcement officials and child advocates say has exploded with the Internet.