An apparent instant clogging of a bathtub drain is almost always caused by a massive accumulation of hair, pet fur, and bath debris that was previously snagged along the upper walls of the drain shoe and suddenly dislodged all at once by heavy water flow, forming an impenetrable barrier across the P-trap inlet. Alternatively, dropping a large foreign object down the open drain will create an immediate, total blockage. Verification: Remove the drain stopper to check the upper pipe opening for visible lodged debris.
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