Which condition may lead to a decreased rate of breathing?

A head injury can lead to a decreased rate of breathing due to changes in brain function and regulation of respiratory patterns. The brainstem, which controls automatic functions such as breathing, can be adversely affected by injuries to the head. Damage in this area can disrupt normal respiratory drive, potentially leading to bradypnea, which is a slower than normal rate of breathing.

In contrast, severe anxiety typically results in hyperventilation, allergic reactions can cause airway swelling leading to increased work of breathing rather than a decreased rate, and asthma exacerbations usually cause wheezing and difficulty in breathing, which often requires increased effort rather than a reduced respiratory rate. Therefore, brain injuries are critical in how they can directly affect the mechanisms of respiration, leading to the correct answer.