The nurse is examining an 8-year-old boy with tachycardia and tachypnea. The nurse anticipates which test as most helpful in determining the extent of the child's hypoxia?

A. Pulmonary function test: Pulmonary function tests measure lung volumes, capacities, and airflow to assess chronic respiratory conditions. They are not practical or rapid for assessing acute hypoxia in a child presenting with tachycardia and tachypnea. B. Pulse oximetry: Pulse oximetry provides a noninvasive, immediate measurement of arterial oxygen saturation, allowing rapid detection of hypoxemia. It is highly useful in evaluating the extent of hypoxia in an acutely ill child and guiding supplemental oxygen therapy. C. Peak expiratory flow: Peak expiratory flow monitoring measures the speed of exhalation, primarily used to monitor asthma control. It does not directly quantify oxygenation or hypoxia. D. Chest radiograph: A chest X-ray can identify structural or pathological lung changes, such as pneumonia or atelectasis, but it does not provide direct or real-time information about oxygen saturation or the severity of hypoxia.