**Question:** All of the following statements about El-Tor vibrios are true, except:

A. El-Tor vibrios are less virulent than classical Vibrio cholerae strains.
B. They cause less severe cholera cases compared to classical strains.
C. They were first isolated in Egypt by El Tor Niyyati and Shimozono in 1905.
D. They are less resistant to environmental factors than classical strains.

**Correct Answer:** D. El-Tor vibrios are less resistant to environmental factors than classical strains.

**Core Concept:**
El-Tor vibrios are a biotype of the bacterium Vibrio cholerae. They are responsible for causing cholera and share many similarities with classical Vibrio cholerae strains. However, one key difference lies in their resistance to environmental factors, such as temperature and desiccation.

**Why the Correct Answer is Right:**
El-Tor vibrios gained their name from the fact that they were first isolated in Egypt by El Tor Niyyati and Shimozono in 1905. Unlike classical Vibrio cholerae strains, El-Tor vibrios are less resistant to environmental factors, which contributes to their increased virulence and ability to cause more severe cholera cases. This reduced resistance allows them to survive better in the human intestines, leading to increased transmission and severity of the disease.

**Why Each Wrong Option is Incorrect:**
A. Although the virulence of El-Tor vibrios might be considered lower than classical strains, they are equally pathogenic and cause cholera cases with similar severity.
B. The severity of cholera cases caused by El-Tor vibrios is not necessarily less severe compared to classical strains. Both biotypes can cause severe cholera with potential life-threatening dehydration and electrolyte imbalance.
C. El-Tor vibrios were first isolated in Egypt, not because they are less resistant to environmental factors. The correct statement should be: "El-Tor vibrios were first isolated in Egypt."

**Clinical Pearl:**
Understanding the differences in virulence factors between different biotypes of Vibrio cholerae is essential for clinical practice. While El-Tor vibrios and classical strains cause similar disease presentations, the reduced environmental resistance of El-Tor vibrios allows for increased transmission and disease severity. This highlights the importance of environmental factors in the epidemiology of cholera, which is a waterborne disease.