Which of the following is a contraindication to thermotherapy?

The correct choice highlights a significant safety concern regarding thermotherapy, which involves the application of heat to treat various physical conditions. Thermotherapy can raise the temperature of tissues, which can increase metabolic activity, improve circulation, and promote healing. However, when a patient lacks thermal sensation, they are unable to detect temperature changes in the affected areas. This absence of sensation poses a risk of burns or thermal injury, as the individual may not realize that the temperature is excessive or harmful. Consequently, applying heat in such cases could lead to significant complications.

In contrast, improved circulation is generally a desired outcome of thermotherapy, chronic pain might benefit from its use, and muscle tension can often be alleviated through the application of heat. Therefore, the lack of thermal sensation stands out as a contraindication, as it directly impacts the safety of using thermal agents.