A 62-year-old female with impaired renal function is prescribed sulfamethoxazole/trimethoprim for a urinary tract infection. Which electrolyte disorder is most likely to occur?

• A. Hyperkalemia
• B. Hypermagnesemia
• C. Hypercalcemia
• D. Hyperphosphatemia

Answer: (B)

The key idea is that trimethoprim, a component of sulfamethoxazole/trimethoprim, can act like a potassium-sparing agent in the kidney. It blocks the epithelial sodium channel (ENaC) in the collecting ducts, which reduces sodium reabsorption and lowers the lumen-negative potential that normally drives potassium secretion. As a result, potassium is not secreted into the urine as effectively, so it accumulates in the body. This effect is especially noticeable in patients with impaired kidney function, where the ability to excrete potassium is already reduced. So the most likely electrolyte disturbance is hyperkalemia, and clinicians should monitor potassium levels, particularly in older adults or those taking other medications that raise potassium (like ACE inhibitors or ARBs). Hypermagnesemia, hypercalcemia, and hyperphosphatemia are not commonly linked to this antibiotic regimen.