The patient presents with symptoms suggestive of Tumor Lysis Syndrome (TLS) following chemotherapy for Acute Lymphoblastic Leukemia (ALL). Key indicators include vomiting, decreased urine output, abdominal pain, tetanic spasms, and ECG findings of QTc prolongation. These symptoms point towards significant electrolyte disturbances, particularly hypocalcemia and potentially hyperkalemia and hyperphosphatemia, common in TLS.

Understanding Tumor Lysis Syndrome (TLS)

• TLS occurs when chemotherapy rapidly kills cancer cells, releasing their contents into the bloodstream.
• This leads to metabolic abnormalities: hyperkalemia, hyperphosphatemia, hypocalcemia, and hyperuricemia.
• Severe hypocalcemia, caused by phosphate binding calcium, manifests as tetanic spasms and cardiac issues like QTc prolongation.

Analyzing Treatment Options in TLS

• Normal maintenance intravenous fluid: While hydration is crucial for preventing renal failure in TLS, administering standard isotonic fluids (like Normal Saline) doesn't directly correct severe hypocalcemia. In the presence of severe symptoms like tetany and QTc prolongation, focusing solely on hydration without immediate calcium correction can be detrimental, potentially delaying life-saving interventions.
• Oral phosphate binders: These agents bind phosphate in the gut. In TLS, high phosphate levels worsen hypocalcemia. Binding phosphate helps alleviate this, making binders beneficial, not detrimental.
• Intravenous rasburicase: This drug reduces uric acid levels, another complication of TLS. It addresses hyperuricemia but doesn't directly cause or worsen hypocalcemia. It's part of managing TLS complications.

Identifying the Detrimental Therapy

The most immediate life-threatening complication indicated by the symptoms is severe hypocalcemia (tetanic spasms, QTc prolongation). While IV fluids are a cornerstone of TLS management for hydration and renal protection, providing only Normal maintenance intravenous fluid without addressing the critical drop in ionized calcium is considered detrimental in this acute scenario. It fails to treat the immediate cause of the tetany and ECG changes and could delay necessary calcium replacement therapy.

Therefore, normal maintenance intravenous fluid, when considered as the sole or primary intervention for these acute symptoms of hypocalcemia in TLS, is the detrimental therapy among the choices.