**Core Concept:** EHEC stands for Enterohaemorrhagic Escherichia coli. These are a subset of Escherichia coli (E. coli) strains that can cause severe illness in humans due to their ability to produce Shiga-like toxins (Stx1 and Stx2) which can lead to haemolytic-uraemic syndrome (HUS) and haemorrhagic colitis (HC).

**Why the Correct Answer is Right:** Option C is incorrect because EHEC strains can indeed cause HUS and HC, making it a key feature of EHEC infections. On the other hand, Options A, B, and D are incorrect because these options either do not directly relate to EHEC characteristics or are less significant compared to the key features of EHEC.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** EHEC is a subset of E. coli strains that can cause severe illness in humans, but it does not exclusively cause gastrointestinal (GI) symptoms like diarrhoea and abdominal pain. EHEC infections can also lead to HUS and HC, making this statement incorrect.

B. **Option B:** EHEC strains produce Shiga-like toxins (Stx1 and Stx2) that cause the severe clinical manifestations associated with EHEC infections. This option is incorrect as EHEC is known to cause these symptoms.

C. **Option C:** While EHEC infections can lead to HUS and HC, the statement is incorrect because EHEC is a subset of E. coli strains that can cause severe illness in humans due to its ability to produce Stx1 and Stx2.

D. **Option D:** EHEC infections primarily involve GI symptoms like diarrhoea, abdominal pain, and fever. This statement is incorrect as EHEC is known to cause severe illness due to its ability to produce Stx1 and Stx2, leading to HUS and HC, rather than focusing solely on GI symptoms.

**Clinical Pearl / High-Yield Fact:**

EHEC infections should be considered in the differential diagnosis of patients presenting with GI symptoms like diarrhoea, abdominal pain, and fever. However, the primary focus of EHEC infections is the production of Stx1 and Stx2, which can lead to HUS (haemolytic-uraemic syndrome) and HC (haemorrhagic colitis). These toxins cause endothelial cell damage, leading to the clinical manifestations mentioned above.

**Correct Answer:** C - EHEC infections primarily cause severe illness due to the production of Shiga-like toxins (Stx1 and Stx2) which lead to HUS and HC. The focus of EHEC infections is not on GI symptoms alone but also on the production of toxins causing endothelial cell damage leading to haemolytic-uraemic syndrome (HUS) and haemorrhagic colitis (HC).