**Core Concept**
The cardiac output during exercise is influenced by the integrated response of the autonomic nervous system and the cardiovascular system. In a patient with a transplanted heart, the denervated heart is unable to respond to the normal physiological changes that occur during exercise, such as an increase in sympathetic tone.

**Why the Correct Answer is Right**
The denervated heart in a transplanted patient relies on circulating catecholamines, such as epinephrine and norepinephrine, to increase cardiac contractility and heart rate. This is achieved through the stimulation of beta-1 adrenergic receptors, which increases the intracellular concentration of cyclic AMP and leads to an increase in cardiac output. Additionally, the vasodilatory effects of nitric oxide and prostacyclin help to reduce peripheral resistance and increase cardiac output during exercise.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the denervated heart in a transplanted patient does not respond to the normal physiological changes that occur during exercise, such as an increase in sympathetic tone.
* **Option B:** This option is incorrect because the increase in cardiac output during exercise in a transplanted patient is not due to an increase in stroke volume, but rather an increase in heart rate.
* **Option C:** This option is incorrect because the denervated heart in a transplanted patient does not respond to the normal physiological changes that occur during exercise, such as an increase in parasympathetic tone.

**Clinical Pearl / High-Yield Fact**
In a patient with a transplanted heart, the denervated heart's response to exercise is limited by the lack of sympathetic innervation, and therefore, the increase in cardiac output during exercise is primarily due to an increase in circulating catecholamines.

**Correct Answer:** D. The increase in circulating catecholamines, such as epinephrine and norepinephrine, and the vasodilatory effects of nitric oxide and prostacyclin.