A 38-year-old man presents to the emergency department with acute onset hypotension (BP 85/52 mmHg), tachycardia (HR 118/min), and altered mental status. Clinical examination reveals cold, clammy skin and weak peripheral pulses. The patient reports fever and productive cough for 3 days prior. Which investigation is most appropriate to confirm the diagnosis and guide immediate management in this case of suspected septic shock?