A well respected surgeon

A well respected surgeon was relaxing on his sofaone evening just after arriving home from work.As he was tuning into the evening news, the phone rang. The doctor calmly answered itand heard the familiar voice of a colleague on the other end of the line."We need a fourth for poker," said the friend."I'll be right over," whispered the doctor.As he was putting on his coat, his wife asked, "Is it serious?""Oh yes, quite serious," said the doctor gravely."In fact, three doctors are there already!"Can you understand the joke?