A mentally challenged man testified Wednesday that his foster father molested him for about three years in their Ridge home beginning when he was 10, but acknowledged his earlier denial of the abuse.

The man, whom Newsday is not identifying, was the fourth prosecution witness to accuse Cesar Gonzales-Mugaburu, 60, who later adopted him, of sexual abuse.

During testimony, the 21-year-old man told prosecutors that the first time Gonzales-Mugaburu touched his private parts and had sex with him in his bedroom, it confused him.

“Who’d do that to their own kid,” said the man, who now lives in a group home.

Gonzales-Mugaburu is on trial before Suffolk County Court Judge Barbara Kahn in Riverhead, charged with sexually abusing six boys and a dog. The most serious charge, predatory sexual assault against a child, carries a maximum sentence of 25 years to life.

Gonzales-Mugaburu, who was arrested in January 2016, has pleaded not guilty.

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During questioning by Suffolk Assistant District Attorney Christina Pinnola, the man testified that Gonzales-Mugaburu regularly punished him, including beating him about the head, face and body with his closed fists.

One of those beatings happened, the man testified, while he was shaving and the razor cut him. He said it left a scar near his eye. The man also said Gonzales-Mugaburu forced him to eat his meals alone while standing, and routinely sent him to his room for as long as a week and locked him out of the house, sometimes in the cold, for hours.

Under cross-examination by defense attorney Donald Mates Jr. of Hauppauge, the man said he was outside for five to 15 minutes.

He also told Mates that in March 2015, about 10 months before Gonzales-Mugaburu’s arrest, a Suffolk detective interviewed him.

The man said that he told the detective Gonzales-Mugaburu did not molest him and other boys living in the home, contradicting his testimony Wednesday.

He also called one of his adoptive brothers a liar in the interview, telling the detective that the sibling made up lies about Gonzales-Mugaburu because he was angry at their adoptive father.

That brother is among the other three witnesses to testify against Gonzales-Mugaburu.

In his opening statement March 29, Mates said the accusations against his client were not credible and Gonzales-Mugaburu had not sexually abused anyone. Mates said his client had welcomed more than 100 boys into his home for two decades and had never been charged with abusing the children placed in his care.

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In her opening statement the same day, Assistant District Attorney Laurie Moroff told jurors that during the two decades Gonzales-Mugaburu was a foster parent, he specifically requested that young boys be sent to live with him, she said.

The inconsistencies in the man’s testimony and that of the other witnesses amount to reasonable doubt, Mates said in an interview Wednesday during a trial break.