## **Core Concept**
The World Health Organization (WHO) sets guidelines for drinking water quality to ensure it is safe for consumption. One critical aspect of this quality is the bacteriological standard, which focuses on the absence of harmful bacteria. The guideline aims to protect public health by preventing waterborne diseases.

## **Why the Correct Answer is Right**
The correct answer, **zero E. coli per 100 mL**, reflects the WHO's stringent standard for drinking water. The absence of *Escherichia coli* (E. coli), a common indicator of fecal contamination, in a 100 mL sample of drinking water is crucial. E. coli is a reliable indicator of recent fecal contamination and the possible presence of pathogens. The presence of E. coli in drinking water suggests that the water may be contaminated with fecal matter, which can contain a wide range of pathogens.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify the correct standard.
- **Option B:** This option suggests a countable number of E. coli, which does not align with the WHO's standard of zero E. coli.
- **Option D:** Similar to Option B, this option implies a permissible count of E. coli or another bacterium, which does not match the WHO guideline.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the WHO and other health organizations emphasize the importance of E. coli as an indicator organism for assessing the bacteriological quality of drinking water. The presence of E. coli in drinking water indicates a risk of waterborne pathogens and necessitates immediate action to decontaminate the water supply.

## **Correct Answer:** . Zero E. coli per 100 mL