Yardena Gerwin wasn’t crazy, but some of her doctors thought she might be. Her male doctors.

Something was wrong with Gerwin a few years ago; she was acting strangely and suffering memory lapses, and the male doctors on the staff of the Manhattan hospital where she was being treated suggested sending her to a psychiatric facility.

But some female physicians who examined her suspected something else amiss, finally diagnosing her with a rare autoimmune disease that affected her behavior.

That experience, that encounter with what she calls the men’s “sexist” assumptions about a young woman, inspired her to become an advocate for other young women, said Gerwin, whose illness is now under control. She is especially concerned about those lacking her family’s financial resources and connections — her father is a university professor and her mother is a lawyer. Both stayed by her side in the hospital.

That encounter with what she calls the men’s “sexist” assumptions about a young woman, inspired her to become an advocate for other young women.

Gerwin joined Girl Up, an international empowerment project of the United Nations Foundation that focuses on such issues as combating violence and encouraging education in developing countries. She rose from an entry-level position to head the organization’s local chapter, the N.Y. Coalition. Her duties include fundraising, running and moderating events, and bringing her peers into the activism fold.

“I was known as ‘the feminist’ in my community,” she said.

A resident of the Upper West Side who spent her kindergarten and seventh-grade years studying in Israel, an observant Conservative Jew who led youth services at Darkhei Noam, a congregation on the Upper West Side, a senior at Beacon High School who earlier attended the Abraham Joshua Heschel School, she is also active in several civic organizations. Today, she attends Minyan M’at, a traditional egalitarian minyan at Ansche Chesed.

Recently, she’s advised young members of the #MeToo movement.

Her future: She will begin college at American University in Washington, D.C., in the fall and hopes to follow college with a career in public service or public policy.

Does she resent the male doctors whose stereotypical misdiagnosis about her gender nearly short-circuited her life? No. “Holding a grudge only hurts you.”

Theater bug: The stage captures Gerwin’s interest. She’s gone to “more than 100” Broadway plays since she was a kid. “Plays are a really powerful thing,” said Gerwin, who’s often the youngest person in the crowd. “It’s me … [and] the retirees.”