Exact match with query_string without stemming

How can I do an exact match with the query_string that excludes stemmed words. For example say I search for:

"transient ischaemic attack" it shouldn't bring back "transient ischaemic attacks", it should only bring back "transient ischaemic attack’’ however at the same time I'd still like to be able to use stemming for non-phrases searches such as "attack" which should bring back "attack" and "attacks"