In Expressions^x, Countable nouns^x, Regards^x

Hi I would like to know if the expressions like "So beautiful a house" or "So stupid a person" are possible with countable nouns only! How can I use such expressions with uncountable nouns? For example, 1- It was so fatal (a?) poison. Or any other example with uncountable noun? ...

Hello Mr. Micawber, Thank you for the tip about uncountable and plural countables nouns after of. So the expression 'the activity of users' is correct, and 'the activity of the users' is not because the article has to be placed before singular countable nouns. What do you think about the...