When a patch cable is plugged into a switch, all port activity indicators turn on and users instantly lose network connectivity. Which of the following is the MOST likely diagnosis?

A denial of service attack originates from external or internal hosts generating malicious traffic, not from simply plugging in a patch cable.

An MTU black hole causes large packets to be silently dropped due to PMTUD failure, degrading performance gradually rather than causing instant total outage with all LEDs lit.

A switching loop occurs when a redundant Layer 2 path exists between switches without Spanning Tree Protocol active, causing broadcast frames to circulate endlessly and multiply. This flooding consumes all available bandwidth and switch resources almost instantly, which is why all port indicators illuminate and users lose connectivity the moment the loop-creating cable is inserted.

A defective patch cable would typically result in no link light or intermittent connectivity on a single port, not all-port activity and network-wide outage.