An infant is brought to the emergency room with hemolytic uremic syndrome and thrombocytopenia. Stool sample was cultured and non-lactose fermenting colonies grown on Sorbitol MacConkey agar, which also fermented glucose with gas. Which one of the following bacteria would most likely be isolated from a stool specimen?

**Core Concept:** Hemolytic uremic syndrome (HUS) is a severe complication of Escherichia coli (E. coli) infection, characterized by hemolysis, thrombocytopenia, and acute kidney injury. E. coli O157:H7 is a particularly virulent strain associated with diarrhea and HUS. Thrombocytopenia is a result of platelet destruction, while the lactose-negative colonies on Sorbitol MacConkey agar indicate the absence of Lactose-positive E. coli, which is consistent with E. coli O157:H7 infection.

**Why the Correct Answer is Right:** E. coli O157:H7 is a pathogenic bacterium that produces Shiga toxin. This toxin is responsible for the pathogenesis of HUS. In the diagnostic process, the presence of non-lactose fermenting colonies on Sorbitol MacConkey agar indicates the absence of lactose-positive E. coli, which is a characteristic feature of E. coli O157:H7 infection. The bacterium ferments glucose, producing gas as a byproduct, which is evident in this case.

**Why Each Wrong Option is Incorrect:**

A. E. coli O111:H7 shares a similar pathogenic mechanism and can cause HUS, but it has a different serotype (O111) and lacks the H7 antigen, making it less likely compared to E. coli O157:H7.
B. E. coli O104:H4 was responsible for an outbreak in Europe in 2011 but is not commonly associated with HUS in the U.S.
C. E. coli O26:H11 is another serotype with a similar pathogenic mechanism, but it differs in serotype (O26) and lacks the H11 antigen, making it less likely compared to E. coli O157:H7.
D. E. coli O121:H7 shares a similar pathogenic mechanism, but it has a different serotype (O121) and lacks the H7 antigen, making it less likely compared to E. coli O157:H7.

**Clinical Pearl:** A high index of suspicion and timely diagnosis are crucial in treating HUS, as it can progress rapidly to severe complications and organ failure. Early initiation of appropriate antibiotics and supportive care can enhance recovery rates.

✓ Correct Answer: D. E. coli O157/H7