**Core Concept**
The management of an 18-month-old child presenting with fever, respiratory difficulty, and lethargy requires careful assessment to determine the underlying cause. The child's symptoms are suggestive of a respiratory infection, which can progress to pneumonia or other complications if not managed promptly and appropriately.

**Why the Correct Answer is Right**
The child's respiratory rate of 46 bpm is elevated, indicating respiratory distress. However, the absence of chest retractions suggests that the child is not in severe respiratory distress at this moment. Given the child's age and weight, the most appropriate initial management would be to administer oxygen therapy to prevent hypoxia and provide supportive care. The child should also be evaluated for potential bacterial causes of pneumonia, such as Streptococcus pneumoniae or Haemophilus influenzae type b (Hib), and treated with antibiotics if necessary.

**Why Each Wrong Option is Incorrect**
**Option A:** Administering antibiotics without a confirmed diagnosis of bacterial pneumonia would be premature and may contribute to antibiotic resistance.
**Option B:** Intubation and mechanical ventilation may be necessary if the child's condition deteriorates, but it is not the initial management step.
**Option C:** Chest X-ray may be useful for diagnosing pneumonia, but it should not be the first step in management. The child's clinical presentation and respiratory rate already suggest respiratory distress.

**Clinical Pearl / High-Yield Fact**
In children under 2 years of age, pneumonia is a leading cause of morbidity and mortality. The World Health Organization recommends the use of antibiotics in children with signs of pneumonia, such as fast breathing, cough, and difficulty breathing.

**Correct Answer: B.**