The US, if they really wanted to, could make it problematic enough for Russia that they might want to engage on this.

Russian doesn't gain anything from having it's citizens attack US hospitals.

Interpol notice - at least that'll restrict their movements

Sanctions and a million other things. Its not "let them commit crime against us" or nuclear war. There's middle ground of course.

Realistic option: Sanctions.

Extreme option: Follow the precedent Russia has set, synthesize some Novichok, and have someone pay them a visit.