**Core Concept**
Tumor lysis syndrome (TLS) is a life-threatening condition that occurs when a large number of cancer cells die rapidly, releasing their contents into the bloodstream. This leads to hyperkalemia, hyperphosphatemia, hypocalcemia, and acute kidney injury due to uric acid nephropathy.

**Why the Correct Answer is Right**
The initial treatment of TLS involves addressing the metabolic derangements caused by cell lysis. Allopurinol is a xanthine oxidase inhibitor that reduces uric acid production by inhibiting the enzyme xanthine oxidase. This prevents the formation of uric acid, which can crystallize in the kidneys and cause damage. By administering allopurinol, clinicians can prevent the development of uric acid nephropathy and reduce the risk of acute kidney injury.

**Why Each Wrong Option is Incorrect**
**Option A:** **Furosemide** is a loop diuretic that can actually worsen TLS by increasing uric acid excretion, which can lead to uric acid nephropathy.
**Option B:** **Insulin and glucose** may be used to manage hyperkalemia, but they are not the initial treatment for TLS.
**Option C:** **Hemodialysis** may be necessary in severe cases of TLS, but it is not the initial treatment.

**Clinical Pearl / High-Yield Fact**
Remember that TLS is a medical emergency that requires prompt recognition and treatment to prevent renal failure and other complications. The key is to prevent uric acid nephropathy by administering allopurinol and addressing hyperkalemia and hyperphosphatemia with supportive care.

**Correct Answer:** A. Allopurinol.