**Core Concept**
Thiazide diuretics are a class of medications used to treat hypertension and edema. They work by inhibiting the sodium-chloride cotransporter in the distal convoluted tubule of the nephron, leading to increased sodium and water excretion.

**Why the Correct Answer is Right**
Thiazide diuretics can cause hypokalemia due to increased potassium excretion in the urine. This occurs because the increased sodium delivery to the distal convoluted tubule leads to increased potassium secretion in exchange for sodium reabsorption. As a result, potassium levels in the blood decrease. This can lead to muscle weakness, fatigue, and cardiac arrhythmias.

**Why Each Wrong Option is Incorrect**
**Option A:** Thiazides can actually increase calcium reabsorption in the distal convoluted tubule, leading to hypercalcemia, not hypocalcemia.
**Option B:** Thiazides can cause hyperglycemia due to increased glucose reabsorption in the proximal convoluted tubule, but this is not the most well-known side effect.
**Option C:** Thiazides can cause hyponatremia due to increased water reabsorption, but this is not the correct answer.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "THIAZIDE" to help you remember the common side effects of thiazide diuretics: Thirst, Hypokalemia, Increased urination, Altered mental status, Zinc deficiency, Increased glucose, Edema.

**Correct Answer: A. Hypokalemia**