Q. A 3-day-old newborn male infant is brought to the emergency department with complaints of irritability, sweating, and tremors. His parents report the patient has appeared weak and is not latching or feeding well. The birth history and pregnancy were uncomplicated and the delivery was a spontaneous vaginal delivery at 39 weeks. His birth weight was 3.2 kg (7 lb) and his Apgar scores were 9 and 1 minute, and 9 at 5 minutes. On physical examination, the neonate is active and responsive, though poor muscle tone is appreciated. He weighs 3.1 kg (7 lb), and his head circumference is 36 cm (14 in), and length is 50 cm (19 in). The fontanelles are both soft and slightly depressed, and his cranial sutures are intact and easily palpable. His eyes are symmetrical and reactive to light, with no evidence of retinal hemorrhage. The patient’s ears are symmetrical and have good mobility, with no evidence of congenital deformity. His nose is also symmetrical, with no evidence of congenital abnormality, and the palate is intact and easily visualized. His mouth is patent and moist, with no evidence of oral thrush. His neck is supple, with no evidence of cervical lymphadenopathy or thyromegaly. Auscultation of his chest reveals good symmetry, with good expansion, and clear breath sounds bilaterally. His cardiac assessment demonstrates regular rate and rhythm, with no murmurs appreciated. His abdomen is soft and nontender, with no masses or hepatosplenomegaly palpated, and the umbilicus is midline with evidence of good healing, and no signs of infection. Genital exam reveals normal male genitalia, with good testicular descent and no signs of hypospadias or epispadias. His extremities were well-perfused, with good pulses and good range of motion. His spine is straight, and his skin is warm and dry, with good turgor. You order blood work, which reveals a blood glucose level of 50 mg/dL, sodium of 125 mEq/L, and potassium of 6.0 mEq/L. You suspect a congenital problem and review the results of his newborn screening assessment. Which of the following tests commonly performed as a component of newborn screening programs will be most helpful in diagnosing this patient’s disorder?