Final answer:
The discontinuation of ultrasound therapy on the right sacroiliac joint is likely due to non-improvement of symptoms, patient preference, or a direct order from a physician, with the fracture of the left third metacarpal being unrelated.
Step-by-step explanation:
The reason for discontinuing ultrasound (US) in the area of the right sacroiliac (SI) joint may vary depending on the context provided by non-clinical factors such as patient preference or clinical directives such as a verbal order from a physician. However, from a clinical standpoint, the discontinuation of US would typically be due to either lack of improvement in symptoms or medical reevaluation of the patient's condition. In the question given, the most likely reasons would be patient preference, no improvement in symptoms, or a verbal order from Dr. Smith. The fracture of the left third metacarpal is unrelated to the SI joint and would not be a valid reason to discontinue US treatment in that particular region.