A Derby nurse practitioner identified as the state’s highest Medicare prescriber of potent narcotics has admitted taking kickbacks from a drug company in exchange for prescribing pain medication. Heather Alfonso, 42, of Middlebury, pleaded guilty Tuesday in U.S District Court in Hartford to receiving $83,000 in kickbacks from January 2013 until March 2015 from an unnamed pharmaceutical company that makes a drug used to treat cancer pain. In pleading guilty, Alfonso admitted that the money she was paid influenced her prescribing of the drug, according to the U.S. Attorney’s Office for Connecticut, which is handling the case. The charge of receipt of kickbacks in relation to a federal healthcare program carries a maximum term of imprisonment of five years and a fine of up to $250,000. U.S. District Judge Michael P. Shea scheduled sentencing for Sept. 17, 2015.