Final answer:
The Fecal float and smear is the recommended test for a cat with severe diarrhea and rectal prolapse, aiming to identify causative agents through direct fecal specimen analysis over multiple days.
Step-by-step explanation:
The recommended diagnostic test for a cat presenting with severe diarrhea and rectal prolapse is a Fecal float and smear. This test allows for the direct examination of fecal specimens which can be crucial for diagnosing various conditions that may cause such symptoms in cats. Diagnosis often involves collecting stool samples on multiple days, as a one-time sample may not be sufficient to detect intermittent shedding of parasites or bacteria that might be causing the illness.
Additionally, specialized tests such as a stool O&P exam, acid-fast staining, immunoassays, and molecular analysis like PCR might also be helpful in identifying certain pathogens. It's important to note that while other tests such as complete blood count, urinalysis, or abdominal ultrasound might be part of a comprehensive evaluation, they are not specifically tailored to the identification of causative agents of diarrhea and rectal prolapse as the fecal examination is.