I have a tenant in one of the units of a duplex in which I live. I am in the process of selling the duplex (Pittsburgh, PA). I had a realtor coming in to take pictures of the house and saw that my tenant had a snake in one of the rooms (of course hidden). On the pet addendum, this tenant mentioned that she had a cat and a dog.  It does say on the lease that no other pet should be brought in on the property other than the ones mentioned. Now I don't know for how long she had that snake for. I didn't see it either given I don't go to her property. She clearly broke the lease. Please recommend the actions steps that I can/should take. 

There is only one action to take.  Your lease should describe the remedies for the resident's violation.  Follow the language of the lease.

Originally posted by @Greg Scott :

There is only one action to take.  Your lease should describe the remedies for the resident's violation.  Follow the language of the lease.

I didn't mention in the lease what happens if a lease is violated. Will do it in future but what are my options now?

If there has been a lease violation, they are in breach of contract (the lease being the contract).  Provide them a letter that they must resolve the situation within X days.  You should seek advice of a real estate attorney for your jurisdiction.

Originally posted by @Greg Scott :

If there has been a lease violation, they are in breach of contract (the lease being the contract).  Provide them a letter that they must resolve the situation within X days.  You should seek advice of a real estate attorney for your jurisdiction.

Thank you Greg for your help.