**Core Concept**
The most probable cause of diarrhoea after eating raw shellfish is an infection caused by a specific type of bacteria, which can be ingested through contaminated food. This condition is often associated with the consumption of undercooked or raw seafood, particularly shellfish.

**Why the Correct Answer is Right**
The correct answer is Vibrio vulnificus, a Gram-negative bacterium that can cause diarrhoeal illness in humans. Vibrio vulnificus is naturally found in coastal waters and can be ingested through the consumption of contaminated shellfish. This bacterium can cause a range of symptoms, including diarrhoea, abdominal cramps, and vomiting, particularly in individuals with weakened immune systems. The pathogenesis of Vibrio vulnificus involves the production of toxins that can damage the intestinal lining and lead to the symptoms associated with the infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Norovirus is a common cause of diarrhoeal illness, but it is not typically associated with the consumption of raw shellfish. Norovirus is primarily spread through the fecal-oral route and is often linked to outbreaks in closed environments such as hospitals and cruise ships.
**Option B:** Salmonella is another bacterium that can cause diarrhoeal illness, but it is not typically associated with the consumption of raw shellfish. Salmonella is often linked to the consumption of undercooked poultry and eggs.
**Option C:** E. coli is a common cause of diarrhoeal illness, but it is not typically associated with the consumption of raw shellfish. E. coli can be found in a variety of foods and can be spread through the fecal-oral route.
**Option D:** Giardiasis is a parasitic infection that can cause diarrhoeal illness, but it is not typically associated with the consumption of raw shellfish. Giardiasis is often linked to the consumption of contaminated water and is a common cause of traveller's diarrhoea.

**Clinical Pearl / High-Yield Fact**
Vibrio vulnificus is a serious infection that can have severe consequences, particularly in individuals with weakened immune systems. It is essential to handle and cook shellfish safely to prevent the risk of infection.

**Correct Answer:** C.