A 28-year-old man with a history of accidental exposure to 2,4-dinitrophenol (DNP) 6 weeks ago presents with persistent unexplained fever (37.8–38.5°C), profuse diaphoresis, and a resting metabolic rate 40% above predicted. Standard investigations (CBC, CMP, blood cultures, thyroid function) are all normal. The clinical team suspects ongoing mitochondrial uncoupling. Which investigation would best assess the current degree of mitochondrial uncoupling and guide therapeutic intervention?