The patient's persistent dry cough is a classic adverse effect of angiotensin-converting enzyme (ACE) inhibitors (e.g., lisinopril, enalapril). ACE is responsible for the breakdown of bradykinin. Inhibition of ACE leads to an accumulation of bradykinin, which is thought to cause bronchial irritation and cough. Angiotensin II receptor blockers (ARBs) do not affect bradykinin levels and are a suitable alternative for patients who develop an ACE inhibitor-induced cough. Calcium channel blockers can cause peripheral edema and gingival hyperplasia. Loop diuretics inhibit the Na-K-2Cl cotransporter and can cause electrolyte abnormalities.

The patient was treated with sublingual nitroglycerin for an anginal attack. Nitrates are metabolized to nitric oxide (NO), which stimulates soluble guanylate cyclase to produce cyclic GMP (cGMP). Increased cGMP activates protein kinase G, which leads to the dephosphorylation of myosin light chains in vascular smooth muscle, causing relaxation. This effect is more pronounced on veins than arteries, leading to venodilation, decreased preload, and reduced myocardial oxygen demand. The drop in blood pressure can cause a reflex tachycardia. Increased cAMP is the mechanism of beta-agonists. Decreased calcium is a downstream effect, not the primary mediator.

The constellation of hypokalemia, hypercalcemia, hyperglycemia, and hyperuricemia is characteristic of thiazide diuretics like hydrochlorothiazide. They inhibit the Na-Cl cotransporter in the distal convoluted tubule. In contrast, loop diuretics like furosemide cause hypocalcemia because they inhibit calcium reabsorption in the thick ascending limb. Spironolactone is a potassium-sparing diuretic and would cause hyperkalemia. Lisinopril, an ACE inhibitor, can cause hyperkalemia, particularly in patients with renal insufficiency.

Significant QT prolongation is a hallmark effect of Class III antiarrhythmic drugs (e.g., amiodarone, sotalol, dofetilide, ibutilide). These drugs primarily block the delayed rectifier potassium channels responsible for phase 3 repolarization of the cardiac action potential. This prolongs the action potential duration and, consequently, the QT interval on the ECG, increasing the risk for early afterdepolarizations and torsades de pointes. Sodium channel blockers (Class I) primarily affect the QRS duration. Calcium channel blockers (Class IV) primarily affect the PR interval. Funny current blockers (ivabradine) slow the heart rate without affecting the QT interval.

Class IC antiarrhythmics, such as flecainide and propafenone, are potent sodium channel blockers with slow dissociation kinetics. They are very effective at suppressing ventricular ectopy. However, the Cardiac Arrhythmia Suppression Trial (CAST) demonstrated that these drugs increase mortality in patients with underlying structural heart disease, such as a prior myocardial infarction, due to a significant proarrhythmic risk. Therefore, their use is contraindicated in this patient population. Class IB agents (e.g., lidocaine) are safer in the post-MI setting.

The patient's presentation of arthralgias, myalgias, malar rash, and positive anti-histone antibodies is characteristic of drug-induced lupus erythematosus (DILE). Hydralazine, a direct-acting arterial vasodilator, is one of the most common causes of DILE, along with procainamide. The risk is higher in patients who are slow acetylators. Prazosin is an alpha-1 blocker, clonidine is a central alpha-2 agonist, and minoxidil is another direct vasodilator, but they are not strongly associated with DILE.

The patient is experiencing first-dose orthostatic hypotension, a classic side effect of α1-adrenergic blockers such as prazosin, terazosin, and doxazosin. These drugs are used for both hypertension and BPH. By blocking α1 receptors on vascular smooth muscle, they cause both arteriolar and venous dilation. The venodilation leads to peripheral pooling of blood, reducing venous return and causing a sharp drop in blood pressure upon standing. This effect is most pronounced with the first dose.

Spironolactone is a mineralocorticoid receptor antagonist (MRA). In heart failure, aldosterone levels are elevated and contribute to disease progression. While spironolactone is a potassium-sparing diuretic and helps with fluid and electrolyte balance, its major survival benefit in HFrEF comes from blocking the non-epithelial effects of aldosterone on the heart and blood vessels. Aldosterone promotes myocardial fibrosis, inflammation, and adverse ventricular remodeling. By blocking these effects, MRAs improve long-term outcomes in HFrEF.

Lidocaine is a Class IB antiarrhythmic agent. Class IB drugs exhibit 'use-dependence,' meaning they have a high affinity for sodium channels in the open or inactivated states, but low affinity for channels in the resting state. Ischemic myocardium is partially depolarized, which means more sodium channels are in the inactivated state. This makes lidocaine particularly effective for arrhythmias arising from ischemic tissue, such as post-MI ventricular tachycardia. Its rapid dissociation kinetics mean it has little effect on the QRS complex at normal heart rates. Flecainide (Class IC) and Procainamide (Class IA) have slower kinetics and widen the QRS.

The patient's side effects, gingival hyperplasia and peripheral edema, are characteristic of dihydropyridine calcium channel blockers (e.g., amlodipine, nifedipine). These drugs act by blocking L-type calcium channels on vascular smooth muscle, leading to arteriolar vasodilation. This reduces systemic vascular resistance and blood pressure. Non-dihydropyridine calcium channel blockers (e.g., verapamil, diltiazem) have a greater effect on the heart, decreasing the SA node firing rate, but are less associated with these specific side effects. Nitrates increase cGMP. Thiazide diuretics inhibit sodium reabsorption in the distal convoluted tubule.