**Core Concept**
Thiazide diuretics have a unique effect on calcium levels in the body, which is separate from their primary mechanism of action as diuretics. This effect is due to their ability to decrease calcium reabsorption in the distal convoluted tubule of the kidneys.

**Why the Correct Answer is Right**
Thiazides increase calcium levels in the blood by reducing the amount of calcium excreted in the urine. This is achieved by inhibiting the sodium-calcium exchanger in the distal convoluted tubule, which leads to increased calcium reabsorption in the proximal tubule. As a result, more calcium is available to be reabsorbed into the bloodstream, rather than being excreted in the urine. This effect is particularly pronounced in individuals with low dietary calcium intake.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it suggests that thiazides decrease calcium levels in the blood, which is the opposite of their actual effect.
**Option B:** This option is incorrect because it implies that thiazides work by increasing calcium reabsorption in the distal convoluted tubule, which is not their primary mechanism of action.
**Option C:** This option is incorrect because it suggests that thiazides have no effect on calcium levels, which is not accurate.

**Clinical Pearl / High-Yield Fact**
When prescribing thiazide diuretics, it's essential to monitor patients for signs of hypercalcemia, particularly those with a history of kidney stones or osteoporosis. This is because the increased calcium levels can exacerbate these conditions.

**Correct Answer: D. Thiazides decrease calcium excretion in the urine, leading to increased calcium levels in the blood.**