B737 MUST EXIT RWY WHEN PAX ENTERS LAVATORY AS THEY ARE ENTERING THE RWY CLEARED FOR TKOF.

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AS WE WERE HOLDING SHORT OF RWY 7L AND #1 FOR TKOF; WE SIMULTANEOUSLY RECEIVED 2 CALLS: ONE FROM THE AFT FLT ATTENDANT STATING THAT A PAX HAD JUST GOTTEN UP AND WAS GOING TO THE LAVATORY; AND A SECOND FROM THE TWR CLRING US INTO POS AND HOLD RWY 7L. I WAS OFF OF COM #1 AND SPEAKING WITH THE FLT ATTENDANT WHILE THE CAPT WAS COMMUNICATING WITH THE TWR. BEFORE ACCEPTING THE CLRNC ONTO THE RWY AND WHILE STILL HOLDING; I COMMUNICATED THE PROB TO THE CAPT AND ASKED THE FLT ATTENDANT TO SEAT THE PAX IMMEDIATELY AND INFORM US AS SOON AS THE CABIN WAS SECURE. THE CAPT THEN TOLD THE TWR THAT WE WERE NOW READY AND WE WERE CLRED FOR TKOF RWY 7L. THIS SCENARIO WHILE SIMPLE AND WITH NO VIOLATIONS CREATED A LOT OF CONFUSION AND CHAOS IN THE CABIN; IN THE COCKPIT; AND WITH ATC. I WOULD CONTINUE TO ENCOURAGE THE CABIN CREW TO CONTACT US ASAP IF THEY EXPERIENCE CABIN SECURITY VIOLATIONS TO ALLOW US TIME TO COORD WITH ATC. IN TIME CRITICAL SITUATIONS SUCH AS THIS SCENARIO; EXPLAINING THE NATURE OF THE PROB TO ATC AND SIMPLY MOVING TO THE HOLDING AREA TO REGROUP MAY BE THE BETTER CHOICE IN ORDER TO GIVE EVERYONE TIME TO CLR THEIR HEADS IN PREPARATION FOR A SAFE; FOCUSED DEP.