Final answer:
The preliminary sign of malignant hyperthermia is an unexpectedly high body temperature, often accompanied by signs of increased metabolism such as rapid heartbeat and muscle rigidity. The provided contexts concerning tetanus and UMN disease do not relate to malignant hyperthermia.
Step-by-step explanation:
A preliminary sign indicating malignant hyperthermia, a severe reaction to certain drugs used for anesthesia, can be an unexpectedly high body temperature. However, the information provided seems to conflate symptoms of tetanus and the neurological indications related to clasp-knife response and UMN (upper motor neuron) disease, which is not directly related to malignant hyperthermia. In malignant hyperthermia, signs of increased metabolism such as a rapid heart rate, muscle rigidity, and acidosis may occur. Hyperthermia in this context is an increase in body temperature that is not initiated by the hypothalamus, which is different from the fever where the set point of body temperature is increased in response to an infection or illness.