A 35 year old man with T1DM (type1 diabetes mellitus) is dehydrated with BP of 90/50mmHg. What is the single most appropriate initial investigation?

The key is A. ABG. [Dehydration with low BP points towards probable DKA (diabetic keto-acidosis).

Diabetic ketoacidosis is typically characterized by hyperglycemia over 250 mg/dL, a bicarbonate level

less than 18 mEq/L, and a pH less than 7.30, with ketonemia and ketonuria. In patients with DKA, arterial

blood gases (ABGs) frequently show typical manifestations of metabolic acidosis, low bicarbonate, and

low pH (less than 7.3)].