**Core Concept**
The question is testing the management of a child requiring repeated short-acting bronchodilators, which suggests an underlying airway obstruction, likely asthma or chronic obstructive pulmonary disease (COPD). The correct next line of management involves addressing the underlying pathophysiology and reducing inflammation.

**Why the Correct Answer is Right**
The use of short-acting bronchodilators indicates that the child has airway obstruction, likely due to inflammation and spasm of the airway smooth muscles. The next logical step in management is to add a medication that reduces inflammation, such as inhaled corticosteroids (ICS). ICS works by inhibiting the transcription of inflammatory genes, reducing the production of pro-inflammatory cytokines, and decreasing the number of eosinophils in the airway. This leads to improved lung function and reduced symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as it suggests adding another short-acting bronchodilator, which would not address the underlying inflammation and would lead to potential side effects such as tachycardia and tremors.
* **Option B:** This option is incorrect as it suggests adding a long-acting bronchodilator, which would be more appropriate for patients with severe asthma or COPD who require continuous bronchodilation. However, this would not address the underlying inflammation.
* **Option C:** This option is incorrect as it suggests adding a leukotriene modifier, which would be more appropriate for patients who are not responding to ICS or have a contraindication to ICS.

**Clinical Pearl / High-Yield Fact**
Remember the "Step-Up" approach to asthma management: starting with short-acting bronchodilators, then adding ICS, and finally adding long-acting bronchodilators or biologics if symptoms persist.

**Correct Answer:** D.