Two medical students

Two medical students were walking along the street when they saw an old man walking with his legs spread apart. He was stiff legged and walking very slowly.
One of the students said to his friend, 'I'm sure the poor old man has Petry Syndrome. Those people walk just like that.'
The other student says, 'No I don't think so. The old man surely has Zovitzki Syndrome. He walks slowly and his legs are spread apart, just as we learned in class.'
Since they couldn't agree, they decided to ask the old man.
They approached him and one of the students said to him, 'We're medical students and we couldn't help but notice the way you walk, but we couldn't agree on the syndrome you might have. Could you tell us what it is?'
The old man said, 'I'll tell you, but first you must tell me what you fine medical students think.'
One of the students said, 'I think it's Petry Syn- drome.'
The old man said, 'You thought, but you are wrong'
Then the other student said, 'I think you have the Zovitzki Syndrome.'
The old man said, 'You thought, but you are wrong.'
So they asked him, 'Well, old timer, what exactly do you have?'
The old man said, "I thought it was GAS, but I was wrong."