When a user tries to print a document on a laser printer, vertical lines appear on the printout. Which of the following should the technician replace?

In a laser printer, the imaging drum is a photosensitive cylinder that holds an electrostatic charge to attract toner and form the image. A scratch or a defect on the surface of the drum will consistently fail to hold the charge properly during each rotation. This results in a continuous streak where toner is either incorrectly applied or repelled, creating a distinct vertical line down the entire length of the printed page. Therefore, replacing the drum is the correct solution for this specific print quality issue.

A. Mirror: A dirty or malfunctioning mirror in the laser scanner assembly would typically cause distorted, skewed, or entirely blank sections, not a clean, consistent vertical line.
B. Fuser: A faulty fuser assembly, which uses heat and pressure to bond toner, would cause issues like smudging, ghosting, or toner that easily rubs off the page.
D. Toner: While a defective toner cartridge can cause various print quality problems, the specific symptom of a sharp, repeating vertical line points directly to a physical defect on the drum's surface, not the toner supply itself.

1. Hewlett-Packard (HP) Official Documentation: In the support document "HP LaserJet Printers - Resolving Print Quality Issues," under the section for "Vertical lines or streaks," the primary cause listed is: "The most likely cause is a scratched photosensitive drum inside the print cartridge." (HP Customer Support, Knowledge Base, Document ID: c01491023, "Vertical lines or streaks" section).

2. Brother Industries, Ltd. Official Documentation: In a troubleshooting guide for vertical lines, Brother's support documentation states: "If the print quality has not improved [after cleaning], the drum unit may be damaged. Install a new drum unit." This directly identifies a damaged drum as the cause of vertical lines. (Brother Solutions Center, FAQ ID: 000000004, "Vertical lines or a vertical streak appears on my printouts," Step 4).

3. University of Wisconsin-Madison Courseware/IT Documentation: The Division of Information Technology (DoIT) KnowledgeBase article on "Laser Printer - Troubleshooting Print Quality" identifies "Lines/streaks down the page" as a common problem. The listed cause is "A scratch in the photosensitive drum," and the recommended solution is to "Replace the toner cartridge" (as the drum is often integrated). (University of Wisconsin-Madison, DoIT KnowledgeBase, Doc ID: 7328, "Lines/streaks down the page" row).