**Core Concept**
The Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is a condition characterized by excessive secretion of antidiuretic hormone (ADH) leading to water retention, dilutional hyponatremia, and concentrated urine.

**Why the Correct Answer is Right**
SIADH is diagnosed by clinical presentation, laboratory findings, and radiological evidence. The condition can be caused by various underlying factors, including malignancies, central nervous system disorders, and certain medications. Treatment options for SIADH include fluid restriction, vasopressin receptor antagonists (such as conivaptan and tolvaptan), and loop diuretics.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum sodium levels in SIADH can indeed be less than 135 meq/L, often presenting with hyponatremia. This occurs due to the excessive water reabsorption in the collecting ducts, diluting sodium levels in the blood.
**Option B:** In SIADH, urine sodium levels are typically low or inappropriately concentrated, as the body attempts to conserve water by reducing sodium excretion. This is a key diagnostic criterion for the condition.
**Option D:** The water loading test can be used to diagnose SIADH, as it assesses the body's ability to dilute the urine in response to water loading. In SIADH, the urine does not dilute appropriately, indicating an inappropriately high ADH level.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the water loading test is not a definitive diagnostic tool for SIADH and should be used in conjunction with clinical presentation and laboratory findings.

**✓ Correct Answer: C. Vaptans are new FDA approved drugs for its Rx**