、。
Yeah, it seems like it was a misdiagnosis.
Literal
Yeah, misdiagnosis [copula-だった] seems.
誤診 'misdiagnosis' (誤 'error' + 診 'diagnosis') is a medical compound. みたい 'seems like / apparently' is the casual, spoken alternative to ようだ. うん is the casual affirmative. A brief, relieved conversational exchange — maximum information in minimum words.