A 28-year-old woman, gravida 1, para 0, at 32 weeks' gestation is admitted to the hospital for the management of elevated blood pressures. On admission, her pulse is 81/min, and blood pressure is 165/89 mm Hg. Treatment with an intravenous drug is initiated. Two days after admission, she has a headache and palpitations. Her pulse is 116/min and regular, and blood pressure is 124/80 mm Hg. Physical examination shows pitting edema of both lower extremities that was not present on admission. This patient most likely was given a drug that predominantly acts by which of the following mechanisms?

Direct dilation of the arterioles

Inhibition of β1, β2, and α1 receptors

Inhibition of angiotensin II production

Activation of α2 adrenergic receptors

Inhibition of sodium reabsorption

A 27-year-old G1P0 woman at 25 weeks estimated gestational age presents with a blood pressure of 188/99 mm Hg during a routine prenatal visit. She has no symptoms, except for a mild headache. The patient's heart rate is 78/min. An injectable antihypertensive along with a beta-blocker is administered, and her blood pressure returns to normal within a couple of hours. She is sent home with advice to continue the beta-blocker. The patient returns after a couple of weeks with joint pain in both of her knees and fatigue. A blood test for anti-histone antibodies is positive. Which of the following is the mechanism of action of the intravenous antihypertensive medication most likely used in this patient?

Interference with action of inositol trisphosphate (IP3) on intracellular calcium release

Release endogenous nitric oxide

Inhibition of phosphodiesterase enzyme

A 70-year-old man comes to the physician for the evaluation of pain, cramps, and tingling in his lower extremities over the past 6 months. The patient reports that the symptoms worsen with walking more than two blocks and are completely relieved by rest. Over the past 3 months, his symptoms have not improved despite his participating in supervised exercise therapy. He has type 2 diabetes mellitus. He had smoked one pack of cigarettes daily for the past 50 years, but quit 3 months ago. He does not drink alcohol. His current medications include metformin, atorvastatin, and aspirin. Examination shows loss of hair and decreased skin temperature in the lower legs. Femoral pulses are palpable; pedal pulses are absent. Which of the following is the most appropriate treatment for this patient?

Percutaneous transluminal angioplasty

Direct vasodilators for USMLE Step 3: High-Yield Pharmacology Lessons

Mechanisms & Hemodynamics - Pipe-Opening Principles

Primary Action: Directly relax arteriolar smooth muscle, leading to vasodilation.
Hemodynamic Effect: Marked ↓ in Systemic Vascular Resistance (SVR) and afterload.
- Minimal effect on venous capacitance; preload is largely unaffected.
Compensatory Response: Baroreceptor-mediated stimulation of the sympathetic nervous system.
- Results in reflex tachycardia, ↑ contractility, and ↑ renin release.

⭐ Exam Favorite: The profound reflex sympathetic activation often necessitates co-administration with a beta-blocker to manage the resulting tachycardia and increased myocardial oxygen demand.

Arterial Vasodilators - The 'Artery-Only' Agents

Hydralazine
- Mechanism: ↑ cGMP → smooth muscle relaxation. Primarily dilates arterioles, reducing afterload.
- Metabolism: Acetylation in the liver. Status (slow vs. fast acetylator) affects drug levels and risk of toxicity.
- Use: Severe HTN, hypertensive emergencies (incl. pregnancy), co-administered with nitrates for heart failure.
- Adverse Effects: Reflex tachycardia, fluid retention/edema, headache, angina. Drug-induced lupus-like syndrome.
Minoxidil
- Mechanism: Opens ATP-sensitive K+ channels ($K_{ATP}$) in vascular smooth muscle → hyperpolarization & relaxation.
- Use: Severe, treatment-refractory hypertension.
- Adverse Effects: More potent than hydralazine. Causes significant reflex tachycardia and fluid retention. 📌 Hypertrichosis (hirsutism) - exploited for topical use (Rogaine) in alopecia.

⭐ High-Yield: Hydralazine-induced lupus is more common in patients who are slow acetylators and is associated with anti-histone antibodies.

Mixed Vasodilators - The Emergency Responders

Sodium Nitroprusside
- MOA: Directly releases Nitric Oxide (NO) → ↑ cGMP → potent vasodilation of both arterioles and venules.
- Hemodynamic Effects: ↓↓ Preload (venous pooling) & ↓↓ Afterload (reduced SVR).
  - Maintains stroke volume; ↑ heart rate (reflex tachycardia).
- Use: Hypertensive emergencies (e.g., malignant hypertension, aortic dissection). Administered via continuous IV infusion.
- Metabolism & Toxicity:
  - Metabolized to cyanide and NO.
  - ⚠️ Cyanide Toxicity: Risk ↑ with renal impairment or prolonged (>24-48 hrs) / high-dose infusions. Presents with altered mental status, seizures, lactic acidosis.
  - Antidotes: Sodium thiosulfate (provides sulfur groups), hydroxocobalamin (binds cyanide), sodium nitrite (induces methemoglobinemia).
- 📌 Mnemonic: NitroPrusside → NO Provider.

⭐ Exam Favorite: Nitroprusside can cause coronary steal syndrome by dilating coronary arterioles, potentially shunting blood away from ischemic areas.

Sodium Nitroprusside: Mechanism & Cyanide Toxicity

High‑Yield Points - ⚡ Biggest Takeaways

Hydralazine and Minoxidil are primarily arteriolar vasodilators, causing a significant decrease in afterload.

The profound drop in blood pressure triggers reflex tachycardia and fluid retention, necessitating co-administration with β-blockers and diuretics.

Remember drug-induced lupus for Hydralazine and hypertrichosis for Minoxidil.

Sodium nitroprusside dilates both arteries and veins for hypertensive emergencies but carries a risk of cyanide toxicity.

Fenoldopam, a D1 agonist, uniquely increases renal blood flow, making it ideal for hypertensive crises with renal insufficiency.

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Direct vasodilators