By Natasja Sheriff

NEW YORK (Reuters) - Prosecutors rested their case in the trial of a former grocery worker accused of the 1979 choking murder of a New York City boy, wrapping up on Friday with testimony of a medical examiner describing how strangulation and asphyxiation affect the body.

Pedro Hernandez, charged with kidnapping and murdering 6-year-old Etan Patz, confessed in 2012 to police that he choked the boy, stuffed him in a box and left him in a New York alley.

Patz vanished as he walked alone for the first time to a school bus stop in his Manhattan neighborhood on May 25, 1979. His disappearance sparked a national movement to find missing children, with his picture one of the first to appear on milk cartons.

In his confession to police, which the defense says was coerced, Hernandez said the boy was alive and moving when he put him in a box.

Dr. Michele Slone, Deputy Chief Medical Officer with the New York City Office of the Chief Medical Examiner, testified that a person could pass out within a few seconds to more than a minute from strangulation, depending on the pressure.

She also said a person could recover within seconds if the pressure stopped but could suffer irreversible brain damage and death if the pressure continued.

Defense attorney Alice Fontier pressed the doctor on whether the style of choking Hernandez described in his confession may not have proved fatal to the boy.

The medical expert responded by saying there were "too many different scenarios" for her to answer definitively.

Patz has never been found. He was declared dead in 2001.

Prosecutors kicked off their case in late January in state Supreme Court in Manhattan. The defense is expected to take two or three weeks to present its case.

Defense attorneys say Hernandez is mentally ill, concocted his confession under police pressure and suffers hallucinations.

"There are many things about his confession, even the manner in which he says that he did this, that are unusual and not very likely to have actually happened," Fontier said outside the courtroom.

Hernandez was questioned and arrested in 2012 after police got a tip that he confessed to the crime to a church prayer group in New Jersey.

He told police he lured the boy into the Manhattan grocery store where he worked with the offer of a soda.

(Editing by Ellen Wulfhorst and Mohammad Zargham)