WASHINGTON—Wrapping her leather-gloved hand around the throttle of a vintage Harley-Davidson, Treasury Secretary Janet Yellen folded up a picture of a highly volatile new cryptocurrency, revved her bike’s engine, and sped away in pursuit, sources at the scene reported Thursday. “You can run, but you can’t stay decentralized forever,” said Yellen, who committed the digital coin’s symbol to memory before she strapped on a fire-red helmet and raced off toward the horizon, leaving a plume of dirt and fumes behind her in the Treasury Department’s parking lot. “I’m coming for you, you son of a bitch. You’ve seen what happened to Dogecoin, to TerraUSD. Your freewheeling days of wild fluctuation won’t last forever. Not if Janet Yellen has any say in the matter.” Reached for comment, Yellen acknowledged she did not have any say in the matter, but hoped Congress would pass regulations to guard against the risks of digital currencies.