To select the correct word to complete the sentence "The intricate operation on the patient required the expertise of a/an __________ surgeon.", we need to understand the context and choose the most appropriate adjective for "surgeon".

1. Understanding the Context:
   • The sentence describes a situation where a surgeon with a particular type of expertise is needed for a complex surgery.
   • We are looking for a noun or an adjective that best describes a professional’s qualification or field of specialization.
2. Evaluating Options:
   1. specialist: This is a noun used to describe a person who concentrates primarily on a particular subject or activity; a person highly skilled in a specific and restricted field. This fits perfectly in the context of a 'surgeon' with specialized skills or knowledge.
   2. especial: This is an adjective that means 'better or more than usual'. While it is close, it doesn't align with the typical usage when describing someone's profession.
   3. special: This is an adjective which means better, greater, or otherwise different from what is usual. However, it is commonly not used before 'surgeon' when indicating specialization.
   4. specialised: This adjective means to develop in a way most suitable for a particular function. However, it usually describes processes, methods, or areas rather than directly describing 'surgeon' in this context.
3. Conclusion:
   • The word "specialist" is the most appropriate choice because it directly refers to someone who has a specific expertise, which matches the context of needing a specialized professional for a complex surgery.
   • Thus, the correct sentence is: "The intricate operation on the patient required the expertise of a/a specialist surgeon."