Which of the following is not a part of the drainage system?

The fixture vent is not considered a part of the drainage system in traditional plumbing terminology. The drainage system primarily includes components that carry waste and water away from fixtures towards the sewage system or a treatment facility.

The building drain refers to the pipe that collects waste from the various fixtures and conveys it to the building sewer. The building sewer is the pipe that carries waste from the building drain to the municipal sewer line or a private disposal system. A branch, on the other hand, is a pipe that connects individual fixtures to the building drain, allowing multiple fixtures to share the same drainage pathways.

In contrast, the fixture vent's primary role is to ensure proper air circulation within the drainage system to prevent pressure differences that could disrupt the flow of waste. It allows air to enter the drainage system, thereby maintaining the necessary gravity flows and preventing syphoning of trap seals. While essential for the overall function of the plumbing system, it does not transport waste itself, thereby categorizing it differently from the primary components of a drainage system.