Brief Answer:
Blood work and doctors visit mandatory

Detailed Answer:
Hello ma'am and welcome.

Thank you for writing to us.

I have gone through your query with diligence and would like you to know that I am here to help. Weakness in the legs can have various causes to it an especially as you have not mentioned your fiancé to have experienced such a presentation in the past (absent history) and an acute onset could indicate simpler causes lil a low blood sugar (as he might not have eaten well enough or for a long duration of time), change in blood pressure, visual abnormalities, ENT (ear, nose, throat) infection or abnormalities, etc.

But it is important to note that there no exist more serious chronic conditions like multiple sclerosis as well, which can result in such a presentation and quite often. And the presence of a lowered immunity (by infections, weather, poor diet, etc.) can cause an aggravation in the symptoms or presentation.

So in my opinion, a complete blood work and visit to the doctor is warranted by his presentation. It is always best to rule out the worst and then target the simpler causes, find and treat them than living in the dark, I hope you understand.

I wish you and your fiance good health, and a happy life ahead together. I hope you find my response helpful. Please feel free to write back to me for any further clarifications, I would be more than happy to help you.

Best wishes.

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