This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Test for quantification of viral infectivity is 
Select and explain. 
A. Cytopathic effect, B. Plaque assay, C. Electron microscopy, D. Haemagglutination
B: Plaque assay
Answer: B: Plaque assay 
Explanation: Viral assays

Virus content of a specimen can be assayed in two ways

1. With reference to total virus particles-

Electron microscopy
Hemagglutination

2. With reference to the infectious virions only
A. Quantal Assays
→ Indicates the presence or absence of infectious virus e.g

Death of the experimental animal.
Production of haemagglutinin in allantoic fluid.
Appearance of cytopathological effects on cell cultures.

B. Quantitative infective assay

It measures the exact number of infectious particles. Eg:
Plaque assay (monolayer cell culture) (CAM).
Pock assay on chick embryo chorioallantoic membrane.

→ Viruses producing pocks on chorioallantoic membrane are HSV, vaccinia, smallpox (variola), monkey pox, cowpox and camel pox.