The pilot reported that, while taxiing the airplane from the ramp to the runway in nighttime conditions he noted the location of the two sets of taxiway lights that indicate the location of the ramp entrance to taxiway alpha. The pilot then reached down to configure the radio and when he looked back outside to the left, he saw one row of two blue taxiway lights, however he did not see the other set of taxiway lights to the left of the set he initially saw and believed that what he saw was the first row (north side). He immediately applied left rudder input to turn but the airplane suddenly stopped, as the airplane had exited the taxiway ramp and impacted a drainage culvert. The aft section of the fuselage was substantially damaged. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.