**Core Concept**
Triamterene is a potassium-sparing diuretic that inhibits sodium reabsorption in the distal convoluted tubule by blocking the epithelial sodium channels (ENaC). This action reduces sodium and water excretion while conserving potassium, leading to increased extracellular fluid volume and potential electrolyte imbalances.

**Why the Correct Answer is Right**
Triamterene causes muscle cramps due to its potassium-sparing effect, which results in hyperkalemia. Elevated serum potassium levels disrupt normal neuromuscular excitability, leading to muscle irritability and cramps. This is particularly notable in patients with renal impairment or those on concomitant potassium-sparing diuretics. The mechanism is not direct but secondary to hyperkalemia-induced changes in action potential propagation.

**Why Each Wrong Option is Incorrect**
Option A: Triamterene does not improve glucose tolerance; it has no significant effect on insulin sensitivity or glucose metabolism.
Option C: Urea levels are not directly affected by triamterene; urea production is primarily governed by protein metabolism and renal function, not diuretic type.
Option D: Triamterene causes **hyperkalemia**, not hypokalemia, due to potassium conservation in the kidneys.

**Clinical Pearl / High-Yield Fact**
Potassium-sparing diuretics like triamterene increase serum potassium, leading to hyperkalemia — a key side effect that can cause arrhythmias or muscle cramps. Always monitor potassium levels in patients on these agents, especially with concomitant medications like ACE inhibitors or potassium supplements.

✓ Correct Answer: B. Muscle cramps