A patient describes recurrent, intrusive thoughts that he might have left the stove on, causing him significant distress and preoccupation throughout the day, but he does not actually go and check the stove or perform any related action in response. Before revealing the answer: is this a compulsion?

No, this is more consistent with an obsession alone than with a compulsion. A compulsion is specifically the behavioural, motor expression of an underlying urge or fear; here, there is no repetitive action or ritual actually being carried out, only the recurrent, intrusive thought itself and the distress it causes. Without an accompanying behavioural response, this remains an obsession rather than a compulsion, even though the two very often occur together.