What is the single most appropriate initial investigation for a 35-year-old man with type 1 diabetes and reduced conscious level?

In a scenario involving a 35-year-old man with type 1 diabetes who presents with a reduced level of consciousness, the most appropriate initial investigation is arterial blood gas.

This choice is crucial because a reduced conscious level in an individual with type 1 diabetes could be indicative of several metabolic emergencies, such as diabetic ketoacidosis (DKA) or severe hypoglycemia. An arterial blood gas analysis provides valuable information regarding the patient's acid-base status, oxygenation, and carbon dioxide levels. Specifically, it can help determine if there is metabolic acidosis, which is common in DKA, and it can also assess the degree of respiratory compensation and overall metabolic function.

While other investigations might provide important information about the patient’s overall health or specific diabetic management (like HbA1c for long-term glycemic control, or liver function tests for liver health), they would not address the immediate need to assess the patient's acute metabolic status and potential complications rapidly. Thus, an arterial blood gas measurement is essential to guide further treatment and management decisions quickly.