**Question:** Fecal contamination of water is evaluated by

A. E. coli count
B. Calcium ions concentration
C. pH value
D. Total dissolved solids (TDS)

**Core Concept:**
The evaluation of fecal contamination in water is crucial for assessing water safety and preventing waterborne diseases. Fecal contamination can introduce pathogenic microorganisms to the water supply, leading to diseases like cholera, dysentery, and typhoid fever. Various methods are employed to determine the presence of fecal contamination, including chemical, biological, and physical methods.

**Why the Correct Answer is Right:**
The correct answer, A. E. coli count, is a biological method used to assess fecal contamination. E. coli, a group of bacteria commonly found in the intestines of warm-blooded animals, is often used as a surrogate marker for fecal contamination. A higher E. coli count indicates the presence of fecal matter, raising concerns about water safety and the potential risk of waterborne diseases.

**Why Each Wrong Option is Incorrect:**

B. Calcium ions concentration (Option B) is a chemical parameter that does not directly relate to fecal contamination. High calcium levels in water may be indicative of certain geological formations or sources, but it does not assess fecal contamination risks.

C. pH value (Option C) is a chemical parameter that can vary significantly due to the source water, location, and other factors. It does not specifically evaluate fecal contamination but rather reflects the overall water quality.

D. Total dissolved solids (TDS) (Option D) is a chemical parameter that measures the total amount of dissolved salts and minerals in water. It is unrelated to fecal contamination assessment and does not provide information regarding the presence of pathogenic microorganisms.

**Clinical Pearl:**

Understanding the different parameters used to assess water quality is essential for healthcare professionals, particularly in areas with limited resources. The combination of biological, chemical, and physical methods helps in comprehensive water quality assessment and ensures proper risk evaluation for waterborne diseases.