High blood pressure, or hypertension, is a prevalent condition in the United States.  One in three adults has hypertension, and approximately 20% do not know that they have it.  If left untreated, high blood pressure can lead to major health issues, including stroke, heart attack, and kidney disease or failure.  Several drugs can help control hypertension, including those shown in Table 1.  These drugs can be classified into different groups based on their mechanism of action.Table 1  Drugs Used to Treat High Blood PressureDrug Mechanism of actionLabetalol Combined α- and β-blockerReserpine Peripheral adrenergic inhibitorValsartan Angiotensin II receptor blockerEprosartan mesylate Angiotensin II receptor blockerVerapamil hydrochloride Calcium channel blockerMinoxidil VasodilatorsCaptopril Angiontensin converting enzyme inhibitorsLabetalol (Figure 1) exists as four stereoisomers and is administered as a mixture of all four.  This drug can be classified as a pseudohybrid drug because two of the isomers, (R,R) and (S,R), exhibit biological activity; the other two isomers, (S,S) and (R,S), are not active.  The (R,R) isomer acts as a nonselective β-adrenergic receptor blocker, and the (S,R) isomer acts as a selective α-adrenergic receptor blocker.Figure 1  Structure of (±)-labetalol and l-epinephrinel-Epinephrine (Figure 1) is the agonist of the α- and β-adrenergic receptors.  The structural motifs that allow l-epinephrine to bind to the adrenergic receptors include an amine separated from an aromatic ring by two carbon units, a hydroxyl group at a chiral center beta to the amine, and two hydroxyl groups on the aromatic ring in the meta and para positions.  With a few structural modifications, the agonist l-epinephrine can be converted into the antagonist labetalol.  Substitution of a hydroxyl on the aromatic ring with an amide group and extension of the methyl group on the amine transforms l-epinephrine into labetalol.  These substitutions keep most of the structural motifs necessary for labetalol to mimic l-epinephrine and bind to the adrenergic receptors. Question 7The number of stereoisomers possible for a given molecule is 2n.  The variable n denotes the number of:A.enantiomers.B.stereocenters.C.diastereomers.D.epimers.