B737-800
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Gate; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft
During taxi out the Flight Attendants alerted the flight deck crew about an odor coming from the aft left side lavatory. The odor was contained only to that lavatory. The odor was described as a wet dog smell. The odor was contained by closing the aircraft lavatory door and locked. The cabin crew did not notice and smoke or fumes; nor did they notice any fire/smoke  warning indications. No crew members or passengers required medical assistance. The decision was made to return to the gate and have Maintenance check out the problem. Operations and Maintenance were notified and passengers deplaned and switched to another aircraft.
B737-800 Captain reported an uneventful ground gate return due to a fume event in the vicinity of the aft lavatory.