The underlined portion is a noun phrase functioning as an appositive in the sentence.
Part of Speech: The underlined portion 'an expert in organ transplants' is known as a noun phrase, specifically a noun (expert) modified by a prepositional phrase (in organ transplants).
Function: In this sentence, the noun phrase 'an expert in organ transplants' functions as an appositive that provides additional information about 'the chief surgeon of the hospital'.
Example: The chief surgeon of the hospital, an expert in organ transplants, routinely spoke at conferences.
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