Cell culture — Which of the following is a semi-continuous human diploid cell line commonly used in virology?

Introduction:
Human cell lines are categorized as primary, semi-continuous (diploid), or continuous (immortalized). This question assesses knowledge of classic lines used for viral isolation and vaccine research.

Given Data / Assumptions:

• HeLa, HEp-2, and KB are well-known continuous lines.
• WI-38 is a human diploid fibroblast strain.
• Semi-continuous lines retain finite lifespan and normal karyotype features.

Concept / Approach:
Diploid strains such as WI-38 and MRC-5 are finite lines with limited passages and near-diploid chromosomes, making them suitable for certain vaccine production steps and for maintaining differentiated properties relative to immortalized lines.

Step-by-Step Solution:

Verification / Alternative check:
Standard virology references list WI-38 and MRC-5 as semi-continuous diploid fibroblasts with finite passage numbers, used historically for certain vaccines.

Why Other Options Are Wrong:

• HeLa: continuous, aneuploid, immortal.
• HEp-2: continuous human epithelial line.
• KB: continuous carcinoma line.

Common Pitfalls:
Assuming all frequently used lines are diploid; many popular lines are immortalized and continuous.

Final Answer:
WI-38