**Question:** Old contamination of water is indicated by -

A. Absence of aerobic bacteria
B. Presence of anaerobic bacteria
C. Presence of coliform bacteria
D. Absence of parasites

**Core Concept:**

Old contamination of water refers to the presence of non-viable microorganisms in water due to previous contamination, which may still pose a risk for potential resuscitation under certain conditions. This contamination can lead to the growth of anaerobic bacteria, which are resistant to oxygen and thrive in the absence of oxygen.

**Why the Correct Answer is Right:**

The correct answer, **B. Presence of anaerobic bacteria,** is relevant in the context of old contamination of water because these bacteria can survive and grow even in the absence of oxygen, indicating the former presence of contamination.

**Why Each Wrong Option is Incorrect:**

A. **Absence of aerobic bacteria (Option A)** is incorrect because absence of aerobic bacteria does not necessarily imply old contamination. Aerobic bacteria require oxygen for growth and would not be present if the water had been previously contaminated.

C. **Presence of coliform bacteria (Option C)** is not the best indicator of old contamination, as coliforms are generally used to assess recent contamination and fecal contamination.

D. **Absence of parasites (Option D)** is not a reliable indicator for old contamination, as the presence of parasites usually indicates recent contamination and does not specifically indicate old contamination.

**Clinical Pearl / High-Yield Fact:**

In clinical settings, understanding the presence of anaerobic bacteria in water samples can help healthcare professionals identify the possibility of old contamination. This knowledge aids in the proper handling, treatment, and potential risk assessment for consuming or using the contaminated water.