HMW9793, these days I can hardly remember anything that happened 5 days ago, never mind 5 years ago. Guess age is catching up with me.
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If indeed my suggestions had helped your DS getting into medical school then I'm most pleased and thank you for the update. I'm glad your DS loves Liverpool, a school which I've high regards for; like said earlier, a nephew went there and also an ex-partner's elder brother trained there as an anaesthetist - a highly specialised post.
As far as I'm concerned, the penultimate year at med school is most crucial - it's a transition stage when a student becomes a real doctor and some may argue it's also the hardest year. Get this year through and you're almost certainly there because the final year is mainly a revision of everything you've learnt in the past years which also involves loads of clinical work at hospitals. Glad that your son is "studying like mad" and I wish him every success.
I don't think 4th year students will be called up to help in the current Covid-19 crisis. For that matter, even final year medics are only allowed to do so much and no more. They certainly are not allowed to "practise" like a doctor because you need to be registered with the GMC to do that. Even after med school you'll only be given a "provisional licence to practise" in your F1 year (i.e. with limitations) followed with a full registration the following year. Before registration all they are allowed to do is like what nurses would do.