By Mark Lamport-Stokes
The American left-hander experienced intense pain in his joints shortly before the U.S. Open at Pebble Beach in June, making it difficult for him to walk.
Since consulting with specialists, however, he quickly began to recover after starting treatment and taking medication.
"Things have been looking great and long term there shouldn't be any issues," Mickelson told reporters on Tuesday while preparing for the year's final major.
"I've got the best kind that you can possibly have, and it's very treatable. I'll probably take this drug for about a year. It's put it in remission. I feel 90 percent."
Psoriatic arthritis is a chronic disease characterized by inflammation of the skin and joints. Stiffness, joint pain and swelling are its main symptoms.
Mickelson, a four-times major winner who clinched the 2005 PGA Championship, first became aware of the symptoms while on holiday with his family in Hawaii eight weeks ago.
"It progressively got worse, and to ultimately where I had to figure something was wrong and had to go get it checked."
"I was concerned about being able to swing a club," he said. "... and I'm surprised at how quickly it's gone away, how quickly it's been able to be managed and controlled.
"Heading into the PGA, I'm probably not as sharp as I would like to be. I didn't play well at the British. But I'm able to work on it. I believe that the game's coming around."
Known for his long-time love of meat burgers, Mickelson sprang another surprise on Tuesday when he announced he had become a vegetarian seven weeks ago.
"Can you believe that?" he smiled. "No, this wasn't recommended by a doctor, I read a book and just thought maybe it will help. And if it helps my situation, I'm all for it."
For a ninth successive tournament, Mickelson has an opportunity to replace Tiger Woods as world number one.
"It's more like a process," said the American, who needs to finish no worse than joint fourth at Whistling Straits to have the chance to take over at the top for the first time.
"If I keep getting better, if I keep working on my game eventually it will happen.
"I've had some poor performances the last couple months. If I can get my game back to where I know it can be, I should be able to accomplish that goal of becoming No. 1."
(Editing by Julian Linden)