THE MYSTERY FILES OF IRENE NG BEHIND NICKELODEON'S `SHELBY WOO' IS A DRIVEN HARVARD SENIOR WHO THINKS ACTING IS A COOL COLLEGE JOB

CAMBRIDGE -- As career dilemmas go, it's a tad unusual for a 23-year-old. Urological surgeon or screen actress? Hmmm.

When Irene Ng graduates from Harvard later this week with better than a 3.8 academic average, she'll hunker down for a couple of months to study for her medical school exams. Then she'll spend about four months shooting 20 more episodes of "The Mystery Files of Shelby Woo," the weekly TV show for kids (Sunday evenings, Nickelodeon) in which she's starred for three seasons as a teenage sleuth. And although she currently considers her acting career -- she's also…