Most victims of infectious mononucleosis recover after a few weeks of

The sentence is trying to communicate a fact. This is something that the simple present tense is great for. Have suffered, however, is present perfect. An unlucky few have suffered for years is likely to be interpreted as "some people have suffered" (in the past or are still suffering). This is not the intended meaning.

Also, you're right about the and. We need a contrast (which the correct option introduces with a but). That is, most people recover quickly, but some may suffer for years.

In this is 'an unlucky few' correct I have doubt in that 'an' is singular and 'few' can be plural.
So this construction seemed wrong to me. Can you please explain this?

Even if you thought there was an issue with singular "few", you still should have ignored it. All five answer choices contain either "a few" or "an unlucky few", so, the word "few" is singular in every choice.





"A few" works like lots of other counting words—e.g., "a dozen", "a hundred", "a million".

These words do take a/an so they're technically singular, but you'll very rarely ever have to use that fact for any reason (such as s-v agreement).
These words are used to count/enumerate things, so they typically appear attached to a plural noun—e.g., "a few NOUNs", "a hundred NOUNs", "a million NOUNs". These whole constructions are plural, as you'd expect from the fact that they're describing multiple NOUNs.