**Core Concept**
The patient's presentation with severe neutropenia (Hb-5 g%, TLC-9000/cc, Neutrophils-3%), lymphoblasts (75%), and prolonged fever suggests a diagnosis of severe neutropenic sepsis, a life-threatening complication of hematological malignancies or aggressive chemotherapy. Effective management requires prompt initiation of broad-spectrum antibiotics to combat the underlying infection.

**Why the Correct Answer is Right**
The use of intravenous (iv) Ciprofloxacin is correct because it is a broad-spectrum antibiotic effective against a wide range of Gram-negative bacteria, including Pseudomonas aeruginosa, which is commonly implicated in neutropenic sepsis. Ciprofloxacin's mechanism of action involves inhibiting bacterial DNA gyrase and topoisomerase IV, thereby preventing DNA replication and transcription. In the context of neutropenic sepsis, iv Ciprofloxacin is administered to rapidly achieve therapeutic levels and combat the infection, which can be life-threatening.

**Why Each Wrong Option is Incorrect**
**Option A:** Oral Ciprofloxacin would be inappropriate in this scenario due to the patient's severe neutropenia, which compromises the integrity of the gastrointestinal mucosa and increases the risk of antibiotic-induced gastrointestinal side effects. Oral antibiotics may also have unpredictable absorption and bioavailability in such patients.
**Option B:** Colony stimulating factor (CSF) would not be the immediate choice in a scenario of neutropenic sepsis, as its administration may be delayed by several hours or days, and it does not address the urgent need for broad-spectrum antibiotic coverage. CSF may be considered later to support bone marrow recovery.
**Option C:** Packed cell transfusion is not indicated in this scenario as the patient's primary issue is severe neutropenia, not anemia. Transfusion of packed red blood cells may even increase the risk of transfusion-related complications.

**Clinical Pearl / High-Yield Fact**
In cases of neutropenic sepsis, it is essential to initiate broad-spectrum antibiotic therapy promptly, as delays in treatment can significantly increase morbidity and mortality. The choice of antibiotic should be guided by local antibiotic resistance patterns and the patient's underlying risk factors.

**✓ Correct Answer: A. iv Ciprofloxacin**