**Core Concept**
The El Tor strain of Vibrio cholerae is a biotype that causes cholera, a diarrheal disease characterized by massive fluid loss and dehydration. This biotype is known for its ability to produce a mild to moderate form of cholera, often referred to as "traveler's diarrhea." The El Tor strain is more virulent than other biotypes and has a higher mortality rate.

**Why the Correct Answer is Right**
The El Tor strain of Vibrio cholerae produces a toxin called cholera toxin, which is responsible for the severe diarrheal symptoms associated with cholera. Cholera toxin is a two-component toxin consisting of an active A subunit and an inactive B subunit. The A subunit, in particular, activates the adenylate cyclase enzyme in intestinal epithelial cells, leading to an increase in cyclic AMP (cAMP) levels. This results in the secretion of chloride ions and water into the intestinal lumen, producing the characteristic diarrhea.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the El Tor strain is not characterized by its ability to cause a mild form of cholera. While it is true that the El Tor strain can cause a milder form of the disease, this is not its primary characteristic.
**Option B:** This option is incorrect because the El Tor strain does not have a lower mortality rate than other biotypes. In fact, it has a higher mortality rate due to its increased virulence.
**Option C:** This option is incorrect because the El Tor strain is not more resistant to antibiotics than other biotypes. While it is true that antibiotic resistance is a problem with cholera, the El Tor strain is not inherently more resistant to antibiotics than other biotypes.

**Clinical Pearl / High-Yield Fact**
The El Tor strain of Vibrio cholerae is highly contagious and can be spread through contaminated food and water, as well as through person-to-person contact. This makes it a significant public health concern in areas with poor sanitation and hygiene.

**Correct Answer:** C.