A 3-day-old newborn is brought to the emergency department due to poor feeding, lethargy, and vomiting. The baby was born at term via an uneventful vaginal delivery. On examination, the infant is hypotonic, with tachypnea and a “sweaty feet” odor. Laboratory studies reveal severe metabolic acidosis, anion gap elevation, and hyperammonemia. Newborn screening results show elevated C5-acylcarnitine levels. Which of the following tests would most likely confirm the diagnosis?
