Final answer:
The anticipated nursing action for a client with a new onset of severe and frequent diarrhea is to collect a stool specimen for analysis to identify the cause.
Step-by-step explanation:
When a client reports a new onset of severe and frequent diarrhea, the nursing action that should be anticipated is to collect a stool specimen. This is an essential step for diagnosing the cause of the diarrhea. The collection and analysis of a stool sample can help to identify possible causative agents, such as bacteria or parasites, and to check for the presence of blood which may indicate a more serious condition. A stool sample is typically analyzed using serological or molecular techniques, such as immunoassays for specific pathogens. Stool samples should be processed promptly to ensure accurate results. In cases where infectious agents are suspected, stool sample analysis is prioritized over more invasive procedures like a colonoscopy or a barium enema, which are not usually the first line of investigation without further evidence suggesting the need for these procedures.