<prompt> A 3 year old boy presents with a 1 day history of loose stools, fever, abdominal cramping, headache, and myalgia. He has no blood in the stool. A careful history reveals that he has several pet tules. Which of the following is most likely the causative agent of his diarrhea?Chlamydia psittaciEntamoeba histolyticaSalmonella sppStaphylococcus aureus  <response>: Salmonella spp., including S. enteritidis and S. typhimurium, produce a gastroenteritis or enterocolitis. Patients with decreased gastric acidity, sickle cell disease, defects in immunity, or children younger than 4 years have a more severe course of disease. Salmonella spp. are carried in nature by animal reservoirs such as poultry, tules, cattle, pigs, and sheep. The incubation period is 8-48 hours after ingestion of contaminated food or water. Chlamydia psittaci produces an interstitial pneumonitis accompanied by headache, backache, and a dry, hacking cough. A pale, macular rash is also found on the trunk Horders spots. Patients at risk include pet shop workers, pigeon handlers, and poultry workers. Entamoeba histolytica produces a diarrhea frequently bloody or heme-positive, right lower quadrant crampy abdominal pain, and fever. Patients frequently have weight loss and anorexia. There is usually a history of travel outside the U.S. Most cases are chronic. Complications include liver abscesses. Staphylococcus aureus produces a self-limited gastroenteritis due to the production of preformed, heat-stable enterotoxins. The incubation period is 16 hours. The toxins enhance intestinal peristalsis and induce vomiting by a direct effect on the CNS. Ref: Ray C.G., Ryan K.J. 2010. Chapter 33. Enterobacteriaceae. In C.G. Ray, K.J. Ryan Eds, Sherris Medical Microbiology, 5e. 