Which medication should be avoided in a client with myxedema?

The correct choice is based on the fact that myxedema, a severe form of hypothyroidism, can significantly affect the metabolism and response to medications. In patients with myxedema, there is a decreased metabolism which can lead to prolonged sedation and respiratory depression when central nervous system depressants, such as pentobarbital sodium, are used. This can cause exacerbation of the condition due to reduced respiratory function and increased risk of complications.

Pentobarbital sodium is a barbiturate that has strong sedative effects, and its use in a patient with myxedema could lead to dangerous levels of sedation given the altered pharmacokinetics in such individuals. Therefore, avoiding this medication helps prevent potential respiratory complications and sedation-related issues.

In contrast, the other medications, though they may have their own considerations, do not pose the same immediate risks as barbiturates in the context of myxedema. For instance, liothyronine is actually used to treat hypothyroidism, furosemide is a diuretic that manages fluid retention without the severe sedative effects, and nitroglycerin can help manage angina without significantly impacting the respiratory function or central nervous system in this specific situation.