Answer:
D. It highlighted the need for valid diagnostic systems.
Step-by-step explanation:
The discovery of the cause of syphilis was important to the field of mental illness because it highlighted the need for valid diagnostic systems. Prior to the discovery of the cause of syphilis, mental illness was often misdiagnosed as syphilis, and vice versa. The discovery of the cause of syphilis led to a better understanding of the differences between syphilis and mental illness and the need to develop better diagnostic systems.