A pediatrician researching tuberculosis, HIV and other infectious diseases was strangled by an exterminator after she questioned his work and was then tied up with equestrian gear and set on fire, a prosecutor said Wednesday in opening statements at his murder trial.

Melissa Ketunuti, 35, had graduated from Stanford University medical school, worked in Botswana and spent about five years at Children's Hospital of Philadelphia as a physician and researcher when she was killed in January 2013.

The trail quickly led to exterminator Jason Smith, the last person known to have visited her three-story row house on a quaint, narrow downtown street. He had not called police to mention his appointment there despite headlines about the case.

"She was alive when I left her!" he blurted out when police arrived at his suburban home in Levittown, prosecutor Peter Lim said Wednesday in opening statements.

Smith ultimately told police during the ensuing interview that he strangled her after she demanded that he spray a foam product throughout her basement. The idea apparently miffed him.

"She wanted the whole basement foamed because of the mice," defense lawyer J. Michael Farrell said, reading excerpts from his client's police statement. "She told me I was incompetent. She said that I shouldn't be an exterminator.

"I tried to leave. ... She stood in my way," he continued. "I strangled her. I tied her up and set her on fire."

Smith bound the physician with her leather riding straps and piled books and papers on top of her before setting her body on fire to cover up the crime, Lim told jurors.

Farrell argued that police coerced a confession out of his client, whom he described as a person of somewhat limited intelligence who had worked for the suburban exterminating company for several years.

"Sometimes innocent people confess to crimes they did not commit," Farrell said.

Smith, now 39, rejected a plea offer that carried a 40- to 80-year term.