**Question:** One of the following statements about hydralazine is not true -

A. Hydralazine is a direct-acting vasodilator that works by blocking the effects of vasoconstrictors.
B. Hydralazine inhibits the enzyme phosphodiesterase.
C. Hydralazine is a sympathomimetic drug that increases heart rate and contractility.
D. Hydralazine is primarily used as a vasodilator in the treatment of hypertension.

**Correct Answer:** C. Hydralazine is a sympathomimetic drug that increases heart rate and contractility.

**Core Concept:** Hydralazine is a vasodilator medication that is commonly used to treat hypertension (high blood pressure). Vasodilators work by relaxing the blood vessels, which reduces overall blood pressure.

**Why the Correct Answer is Right:** Hydralazine is a direct-acting vasodilator, meaning it directly relaxes the blood vessels without affecting the sympathetic nervous system. It works by blocking the actions of vasoconstrictors, which are substances that cause blood vessels to narrow. This relaxation of blood vessels leads to a decrease in blood pressure, making hydralazine an effective medication for hypertension management.

**Why Each Wrong Option is Incorrect:**

A. This statement is correct, as hydralazine is a direct-acting vasodilator that blocks the effects of vasoconstrictors.
B. Hydralazine does not inhibit phosphodiesterase. Phosphodiesterase inhibitors, such as milrinone and sildenafil, are used for different conditions like heart failure and erectile dysfunction, respectively.
C. Hydralazine is a sympathomimetic drug, which means it mimics the actions of the sympathetic nervous system, specifically increasing heart rate and contractility. This is the opposite of what is expected from a vasodilator like hydralazine.
D. While hydralazine is used to treat hypertension, the statement is incorrect because hydralazine is a sympathomimetic drug, as explained above. Sympathomimetics (like adrenaline) increase blood pressure due to their effect on the sympathetic nervous system.

**Clinical Pearl:** Vasodilators like hydralazine should be prescribed with caution in patients with low blood pressure, as they can further decrease blood pressure. Sympathomimetics, on the other hand, can be used to increase blood pressure in patients with high blood pressure, but should be used with caution in patients with low blood pressure as well, as they can lead to dangerous drops in blood pressure. This is an important consideration when prescribing medications to maintain an optimal balance between blood pressure regulation and potential side effects.