You are dispatched to an old apartment building where multiple top-floor units are experiencing slow drainage and occasional backups in their bathtubs and sinks, but not toilets. What is the most likely cause?

Question: You are dispatched to an old apartment building where multiple top-floor units are experiencing slow drainage and occasional backups in their bathtubs and sinks, but not toilets. What is the most likely cause?

Answer options:

• A blockage in the main building sewer
• An individual fixture trap blockage ✅ A partial blockage in the main stack vent or branch vents
• Insufficient water pressure in the building

Correct answer: A partial blockage in the main stack vent or branch vents

Explanation: Slow drainage and backups in multiple top-floor fixtures (bathtubs/sinks) but not toilets often indicate a partial blockage in the main stack vent or specific branch vents servicing those fixtures. This restricts air entry, impairing drainage. A main sewer blockage would affect all fixtures, and individual trap blockages would be isolated to one fixture.