The judge in the Etan Patz murder trial yesterday ruled out a key piece of defense evidence -- a 50-pound box of potatoes designed to discredit defendant Pedro Hernandez's purported confession about disposing of the boy's body.

In his disputed 2012 statement to police, Hernandez said he strangled Patz in a bodega basement in Soho in 1979, stuffed the 6-year-old in a banana box and then hoisted it on his shoulder and up the stairs and 1 1/2 blocks to an alley.

Manhattan Supreme Court Judge Maxwell Wiley said he wouldn't let the defense fill a banana box with potatoes so jurors could experiment with carrying it, even though earlier he let prosecutors show staged pictures of a boy Etan's size crammed in a similar box.

More storiesComplete coverage: Etan Patz case

"That's an objective photograph," explained Wiley. " . . . Lifting a box is going to be a different experience for each individual juror. They're going to have to speculate about how that box felt to Pedro Hernandez on May 25, 1979."

Etan vanished on his way to catch a school bus. His body has never been found. Hernandez, 53, a married father from New Jersey, was working in a bodega next to the bus stop. Hernandez's confession is the only direct evidence he did it.

The defense claims it was a fantasy caused by a mental disorder, and has tried to show Hernandez had some facts wrong -- such as the weather and what Etan wore -- and that some claims are implausible.

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Prosecutors showed the photo of a boy fitting in the box to corroborate the confession, but the defense says Hernandez was 5 feet, 3 inches and 112 pounds and could never have carried it as he said he did.

In testimony yesterday, psychologist Bruce Frumkin became the second defense expert to diagnose Hernandez with schizotypal personality disorder, and testified the mental illness and a low IQ made him vulnerable to confessing falsely under police pressure tactics.

But on cross examination, prosecutor Joan Illuzzi-Orbon challenged Hernandez's low IQ tests by noting he used words like "alleviate" and "fornicate" in taped conversations, and brought up incriminating statements Hernandez made in the 1980s to an ex-wife, a friend and a prayer group.

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What tactics had those people used to trick him? she asked. "I don't know," he responded.

The defense is expected to present a second prong of its case today: an attempt to cast suspicion on Jose Ramos, a convicted pedophile who dated a Patz baby-sitter and was long the prime suspect in Etan's disappearance.

Ramos has said he won't testify, and will invoke his Fifth Amendment right against self-incrimination. Wiley said yesterday he won't tell jurors Ramos is taking the Fifth, which can indicate a person is guilty, but does not necessarily mean that.

Wiley said he would tell jurors Ramos was "unavailable" if the defense wants to offer an explanation for him not appearing.