**Question:** Test for quantification of viral infectivity is

A. plaque assay
B. virus neutralization test
C. PCR (polymerase chain reaction)
D. culture and isolation

**Correct Answer:** A. plaque assay

**Core Concept:** The correct test for quantification of viral infectivity is the plaque assay. A plaque assay is a laboratory technique used to measure the number of viable virus particles in a sample. It is a method to quantify the infectivity of viruses, such as those causing influenza, herpes, or adenovirus infections.

**Why the Correct Answer is Right:** In the plaque assay, a small area of a cell monolayer in a petri dish is infected with the test sample containing viruses. Following the infection, the cell monolayer is treated with a solution that kills all but the cells adjacent to the plaques (growth foci) formed by the infected cells. These cells then divide, forming visible plaques. The number of plaques is proportional to the number of virus particles in the original sample.

**Why Each Wrong Option is Incorrect:**

A. Plaque assay is not the only option for viral quantification. Plaque reduction assay is another method that measures the effectiveness of an antiviral agent by comparing the number of plaques in treated and untreated samples.

B. Virus neutralization test (VNT) is a serological test used to determine the titer of antibodies against a virus in a serum sample. It does not quantify the number of virus particles.

C. Polymerase Chain Reaction (PCR) is a molecular technique used to amplify specific genetic material from a sample. It is not a direct method for quantifying viral infectivity but can be used for gene amplification and identification of viral strains.

D. Culture and isolation is a process of growing and identifying viruses from clinical samples. It does not measure the infectivity or quantify the number of virus particles.

**Clinical Pearl:** The plaque assay is a valuable technique to determine the number of viable virus particles in an infection sample, aiding in understanding viral load and monitoring disease progression or treatment response.