From the FDA Drug Label
Beta-adrenergic agonist medications may produce significant hypokalemia in some patients, possibly through intracellular shunting, which has the potential to produce adverse cardiovascular effects. The most likely test to confirm the diagnosis is A. Serum electrolytes 1, as the patient's symptoms of fatigue, weakness, and proximal muscle weakness of the lower extremities, along with decreased deep tendon reflexes, are consistent with hypokalemia, a known side effect of beta-adrenergic agonist medications like terbutaline.
From the Research
The most likely test to confirm the diagnosis in this case is serum electrolytes (option A). This patient is experiencing symptoms consistent with hypokalemia, a known side effect of terbutaline therapy, as reported in studies such as 2 and 3. The presentation of fatigue, weakness, and proximal muscle weakness of the lower extremities with diminished deep tendon reflexes strongly suggests an electrolyte abnormality, particularly low potassium. Terbutaline is a beta-2 adrenergic agonist used for tocolysis in preterm labor that can cause intracellular potassium shifting, leading to decreased serum potassium levels, as explained in 4 and further detailed in 5. This medication-induced hypokalemia develops rapidly, explaining why symptoms appeared just three hours after initiating high-dose terbutaline. Serum electrolyte measurement would reveal the decreased potassium level, confirming the diagnosis and guiding appropriate treatment with potassium supplementation. The other listed tests would not be as immediately useful in diagnosing this acute medication-induced condition in a previously healthy pregnant woman with a clear temporal relationship between drug administration and symptom onset. It's also worth noting that while terbutaline can be used to manage hyperkalemia in certain contexts, as discussed in 6, the scenario presented here aligns more closely with the side effect profile of terbutaline causing hypokalemia. Therefore, checking serum electrolytes is the most direct and relevant diagnostic step.