Final answer:
Nasal flow decreasing with an increase in respiratory effort typically indicates obstructive apnea, which is caused by a physical blockage of the airway during sleep.
Step-by-step explanation:
If nasal flow decreases with an increase in respiratory effort, this may indicate that airflow is not being properly maintained despite the body's attempts to breathe normally. This condition is likely caused by an obstruction in the airway, suggesting obstructive apnea as the correct answer. Obstructive sleep apnea is characterized by an obstruction of the airway during sleep, leading to periods where breathing is diminished or stops despite the respiratory muscles' efforts. Unlike obstructive sleep apnea, central sleep apnea involves a lack of respiratory effort due to the brain's failure to send appropriate signals to breathe.