What is May-Thurner syndrome? Lauren Boebert’s diagnosis, explained.

May-Thurner syndrome came into the spotlight this week when the election campaign for Rep. Lauren Boebert (R-Colo.) announced that she had been diagnosed with the condition, which can affect blood flow. The hard-right congresswoman was hospitalized after experiencing “severe swelling in her upper left leg” and underwent surgery to remove an acute blood clot and insert a stent, her campaign said Tuesday. She is expected to make a full recovery.

The syndrome is rarely diagnosed, but the vein compression that causes it is present in about 1 in 5 people, according to some estimates. Here’s what...