Which of the following is NOT a mode of superficial heating agents?

Ultrasound therapy is indeed not categorized as a mode of superficial heating agent. Instead, it is typically classified as a deep heating modality. This is due to the way ultrasound generates heat within tissue; it utilizes high-frequency sound waves that can penetrate deeper than just the surface layers of the skin. The energy from ultrasound causes molecular vibration, which increases tissue temperature at a deeper level compared to superficial agents, which primarily heat the skin and underlying soft tissues.

On the other hand, hot packs, paraffin baths, and fluidotherapy all operate by directly applying heat to the skin and effecting a warming effect on more superficial structures. These methods are designed to elevate local tissue temperature, enhancing blood flow, reducing pain, and promoting flexibility in the area treated. Hence, ultrasound therapy stands apart as a unique modality that engages more with deeper tissues, justifying its distinction from the others listed.