I saw this mountain lion track this afternoon in the Trabuco wash about a half mile south of the Santa Margarita Parkway bridge. I guessed that it was the right front paw of a medium-sized cougar, then emailed the photo to tracker Dick Newell to get an expert's opinion. Dick is a naturalist and tracker for the Irvine Ranch Conservancy, and has a terrific web site: OCTrackers.com. Currently Dick is offering tracking classes for volunteer naturalists, rangers, animal control officers, as well as prior graduates of his basic tracking class. He plans on offering beginning tracking classes for the general public in the future, so keep an eye on his web site.
Now through the end of the rainy season is the best time for beginning trackers since tracks are often well-defined in the soft mud or damp sand of the washes. Don't forget to carry a small ruler or tape as standard equipment when you hike; they are a critical in identifying tracks. The experts can identify tracks on dry trails where we may not even notice a track, but that takes years of experience.
By the way, I was partially correct. It is likely the right rear foot of the mountain lion (aka cougar or puma) with a direct register over the right front foot of the animal. Both bobcats and mountain lions often place their rear foot directly where their front foot had been. This is know by trackers as "direct register." Sometimes you will see an indirect register as in this photo I took a few minutes later of a bobcat track. Notice how much smaller the bobcat's track is compared to the lion's.