Step 1: The decision to use steroids versus aspirin in rheumatic fever rests almost entirely on the presence and severity of cardiac inflammation.
Step 2: Carditis, particularly when accompanied by heart failure, is the clear indication for corticosteroids because they give the strongest anti-inflammatory effect on the inflamed myocardium and valves.
Step 3: Milder carditis without failure can be treated with either steroids or aspirin, steroids being favoured, and pure arthritis without cardiac signs is managed with aspirin alone over a course of roughly 12 weeks.
Step 4: Isolated chorea and subcutaneous nodules do not by themselves call for steroids, so the only firm steroid indication among the choices is carditis.
\[\boxed{\text{Carditis}}\]