WASHINGTON — Adam Mosseri, the head of Instagram, will testify before a Senate panel on Wednesday afternoon to defend the social media app from growing bipartisan outrage over its reported harms to young users.

It will be Mr. Mosseri’s first appearance before Congress. He is the highest-ranking official from Meta, the company formerly known as Facebook and the parent company of Instagram, to testify to lawmakers after a whistle-blower leaked internal research that said Instagram had a toxic effect on some teenagers.

Lawmakers are expected to grill Mr. Mosseri about the research, which showed that one in three teenage girls said the app made her feel worse about her body image. He will probably also be questioned about the app’s underlying technology and whether it sends young users into rabbit holes of more dangerous and harmful content. Republican and Democratic lawmakers say they will also confront him about the safety of young users, including the company’s efforts to keep underage users off the site.

“Instagram’s repeated failures to protect children’s privacy have already been exposed before the U.S. Senate,” said Senator Marsha Blackburn of Tennessee, the lead Republican on the Senate Commerce Committee’s panel on consumer protection, which is holding the hearing. “Now, it is time for action. I look forward to discussing tangible solutions to improve safety and data security for our children and grandchildren.”

The hearing begins at 2:30 p.m. Here’s what you need to know ahead of the hearing.

Who is Adam Mosseri?

Mr. Mosseri, 38, is a longtime executive at Facebook and considered a close lieutenant of the company’s chief executive, Mark Zuckerberg. He joined the company in 2008 as a designer and gradually rose in the ranks to run the News Feed, a central feature of the Facebook app.

In October 2018, he was named head of Instagram, weeks after the sudden resignations of the app’s founders, Kevin Systrom and Mike Krieger.

Why the focus on Instagram?

This is the fifth hearing by the Senate consumer protection subcommittee on protecting children online, and executives of TikTok and YouTube have already appeared. But Instagram became the focus of lawmakers after a former Facebook employee, Frances Haugen, leaked internal research that showed some troubling findings about the toxic role Instagram plays in the lives of young users, particularly teenage girls.

Richard Blumenthal, the chair of the subcommittee and a Democrat of Connecticut, said his office had received hundreds of calls and emails from parents about their negative experiences with Instagram. One parent recounted how her daughter’s interest in fitness on Instagram led the app to recommend accounts on extreme dieting, eating disorders and self-harm.

Mr. Blumenthal has homed in on the algorithms that push such recommendations.

“We want to hear straight from the company’s leadership why it uses powerful algorithms that push poisonous content to children driving them down rabbit holes to dark places, and what it will do to make its platform safer,” he said.

What does Congress want to achieve?

Lawmakers, including Mr. Blumenthal and Ms. Blackburn, have proposed stronger data privacy rules aimed at protecting children, greater enforcement of age restrictions and the ability of young users to delete information online.

How might Mr. Mosseri respond?

On Tuesday, Instagram announced new safety features for children. Mr. Mosseri is likely to focus on those new tools, such as a “take a break” feature to limit time spent online. (TikTok has a similar function that appears when users are spending too much time on the app.)

Mr. Mosseri is also expected to focus on more positive research that shows that Instagram can also help young users create relationships online and feel less lonely. And he will express support for some regulations, such as stronger child privacy rules.