Let's see if I remember:
There was a party at Susan's house, where a few people got "tipsy". Lucca started to spill his guts to Abbie after revealing that he got her the snowglobe. She basically said "don't go there" and left with Carter. The Med student then made a pass and went home with Lucca. She then brushed Lucca off at his apartment and didn't sleep with him but stayed over.
The Med student answered the call (at Lucca's) to come into work, on his day off. This prompted rumors that Lucca and the student slept together and Abbie gave the girl a warning. Lucca was still drunk and tired when he went to work, with the student in tow. Seeing Abbie kiss Carter goodbye and getting punched by the elf didn't help his mood.
In a mental fog, Lucca started to make misjudgements about patient care. The Med student was looking for guidance but wasn't getting it, taking the blame instead for some problems.
By the end of the day Lucca was mentally shot and the Med student was trying to understand what was going on. She asked for a ride home. Confronted by the student (regarding the events of the previous night), Lucca threw caution to the wind, didn't slow the car down when requested, and crashed into another car.
In his efforts to then help the victims in the other car he made his final misjudgement of the day ... the Med student was critically injured herself. She finally collapsed and was rushed to the ER. Lucca was left trying to explain what happened and why to his co-workers and himself.