Learn Arabic (Modern Standard): أنت مريض، ولا بد من الدواء. - You are sick, and there is no alternative to the medicine.

It takes a single fatha, making the phrase pronounced 'laa badda'. This is because لا here is the 'absolute negation of category' (لا النافية للجنس). The rule for this structure is that the single, indefinite noun immediately following لا takes the accusative case (mansub) but without nunation (no tanween).

Why isn't there an extra alif at the end of بد if it is in the accusative case?

Usually, an indefinite accusative noun gets an extra alif (like the alif in شكراً). However, because the noun after this absolute لا is built without tanween, we do not add the alif. It is written simply as بد and pronounced with a short 'a' sound at the end.

I've seen لا بد translated as 'must' in other contexts. Is that the same phrase being used here?

Exactly! The noun بد literally means 'escape' or 'way out', so the absolute negation لا بد literally means 'there is absolutely no escape'. Idiomatically, the phrase لا بد من is the standard and most common way in Arabic to express 'must', 'have to', or 'it is necessary'.

Why do we use the preposition من (from) here instead of translating the English word 'to' literally?