I read the story in a hard bound science fiction anthology which I purchased after 1968 and probably in the 1970s. The anthology was rather old when I bought it, so could have been printed in the sixties, fifties, or forties perhaps. Most of my books are now packed up.
In this story a doctor, develops an immortality drug and is accused of murdering a neighboring boy whose dissected corpse is found in his laboratory. It seeems likely that the boy had access to the immortality drug. The boy's first name was Peter, he was about 12, and had a club foot, and the doctor had disliked him since delivering him.
And though the boy's death is not seen, I always had the unpleasant suspicion that the protagonist murdered him.
I sort of remember it as a story by British author S. Fowler Wright.
Here is a link to a list of stories by S. Fowler Wright in case any of the titles seems familiar to anyone.
And the only other story by Wright which I remember reading is "Brain", so I can't eliminate any others.