Selective TTL Pulse Accepting

My setup is basically an arduino uno hooked up to a spectrometer and an LED driver (two separate entities). The spectrometer is sending TTL pulses continuously (once every x seconds) and what I want is that the arduino uno take in these TTL pulses but only send a specific one through to the LED driver. Is there a way to do this - if so, how? Basically I just want one of the TTL pulses to go through to trigger an LED but have the rest not be sent through - is there a way to make the arduino uno accept this single TTL pulse? Thanks!

How is this particular pulse distinguishable from the others?

It will be the first pulse. Check my post in the "Programming" section, that should make more sense. Thanks!