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Polysomnography is the diagnostic test used for sleep apnea because it is a comprehensive evaluation conducted overnight in a sleep lab or at home. This test records various physiological parameters during sleep, such as brain waves, oxygen levels, heart rate, breathing patterns, and muscle activity. By capturing this data, polysomnography can identify interruptions in breathing and other abnormalities that are characteristic of sleep apnea.

The other options, while they may provide some information about respiratory health or sleep patterns, do not have the specificity or capability to diagnose sleep apnea. A chest X-ray can show lung conditions but does not provide direct information about sleep patterns or apnea events. A CT scan of the lungs is useful for visualizing lung structures but again lacks the functionality to assess sleep-related breathing disorders. A sleep diary can help track sleep patterns and potential symptoms but cannot confirm a diagnosis, as it relies on subjective reporting rather than objective measurement.