Republican presidential candidate Donald Trump launched a scathing attack on his most significant rival, retired neurosurgeon Ben Carson, telling an audience in Iowa that Carson described himself as "pathological" in his memoir. To a rather muted assembly of supporters with placards, Donald Trump typically didn't hold back.

"Carson's an enigma. He wrote a book, and he's doing great in Iowa. He's second in the polls. With all these professional politicians, I'm first, Carson's second, and I don't understand it, I really don't understand it. Because he wrote a book and in the book he said terrible things about himself. He said that he's pathological and that he's got basically pathological disease," said Trump.

In his autobiography, Gifted Hands, Carson describes himself as a troubled youth with a temper that led him to attack several of his friends and even his mother, before he turned his life around. Trump compared people who are "pathological" to paedophiles.

"If you're pathological, there's no cure for that folks. OK? There's no cure for that. And I did one of the shows today, and I don't want to say what I said but I'll tell you anyway. I said that if you're a child molester, a sick puppy, you're a child molester, there's no cure for that. There's only one cure − we don't want to talk about that cure. That's the ultimate cure. No, there's two, there's death and the other thing," he said.

Trump also questioned the veracity of an incident from his Carson's youth in which he claimed that he attempted to stab a friend, but that his knife broke on this friend's belt buckle.

"Then here's the beauty of all, he took a knife and he went after a friend and he lunged, he lunged that knife into the stomach of his friend, but low and behold it hit the belt, it hit the belt, and the knife broke. Give me a break, give me a break, give me a break," he said.

Carson has bristled at news media attempts to confirm the stabbing incident and other accounts in his autobiography, saying he is being unfairly targeted.