A random test that a dermatologist might do (4,5)
I believe the answer is:
spot check
'a dermatologist might do' is the definition.
I can't judge whether this defines the answer.
'a random test' is the wordplay.
'a random' becomes 'spot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'test' becomes 'check' (synonyms).
'spot'+'check'='SPOT-CHECK'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for spot check that I've seen before include "Security measure" , "Random test, made without warning" , "Sudden unannounced examination" , "Random or surprise examination" , "From impromptu inspection" .)