**Core Concept**
Chandler's index is a clinical tool used to assess the severity of hookworm infection by measuring the number of eggs per gram of feces (EPG), specifically in *Necator americanus* and *Ancylostoma duodenale*. It helps determine the intensity of intestinal parasitic infestation and is crucial in public health surveillance of soil-transmitted helminths.

**Why the Correct Answer is Right**
Chandler's index is defined as the number of hookworm eggs per gram of feces (EPG), with higher values indicating more severe infection. It is most relevant to *Ancylostoma* and *Necator* species, which are hookworms. The index is used to estimate the burden of infection and guide treatment and control measures in endemic areas.

**Why Each Wrong Option is Incorrect**
Option A: Round worm (Ascaris lumbricoides) does not produce eggs counted by Chandler's index; its egg count is assessed using different methods like the Kato-Katz technique.
Option C: Pin worm (*Enterobius vermicularis*) produces few eggs and is not measured by Chandler’s index; it is diagnosed via tape test.
Option D: Tape worm (*Taenia*) is a cestode with a different life cycle and egg count method; Chandler's index does not apply to cestodes.

**Clinical Pearl / High-Yield Fact**
Chandler's index is specifically for hookworms and is used in endemic regions to estimate infection intensity. Remember: **"Chandler's = Hookworm + EPG"** — never used for other helminths.

✓ Correct Answer: B. Hook worm