Final answer:
Positive pressure ventilation causes an increase in intrathoracic pressure by pushing air into the lungs and raising the pressure within the thoracic cavity.
Step-by-step explanation:
When considering mechanical ventilation, an increase in intrathoracic pressure is caused by positive pressure ventilation (option b). Positive pressure ventilation is a way of providing support for patients who are unable to breathe on their own, by pushing air into the lungs, which increases the pressure within the thoracic cavity. This is contrasted with normal breathing, where inhalation involves expanding the thoracic cavity and creating negative pressure to draw air into the lungs. Exhalation, chest compression, or a pneumothorax can also affect intrathoracic pressure, but positive pressure ventilation is specifically designed to increase it in a controlled manner.