**Core Concept**
The question describes a case of pneumonia in a healthy child, likely caused by a specific pathogen that is common in rural areas during the summer months. The key concept is the identification of the causative agent and the selection of the most appropriate first-line treatment.

**Why the Correct Answer is Right**
The clinical presentation of fever, cough, and lower respiratory symptoms in a healthy child, without upper respiratory tract symptoms, suggests an atypical bacterial infection. The absence of upper respiratory symptoms is a crucial clue, as it points towards an infection caused by Mycoplasma pneumoniae. The peripheral smear examination, which is not specified, might have shown characteristic inclusion bodies or "Morula" in the red blood cells, which are indicative of Mycoplasma infection. The first-line treatment for Mycoplasma pneumoniae infection is macrolide antibiotics, such as azithromycin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify a macrolide antibiotic, which is the preferred first-line treatment for Mycoplasma pneumoniae infection.
**Option B:** This option is incorrect as it suggests a fluoroquinolone antibiotic, which is not typically used as a first-line treatment for Mycoplasma pneumoniae infection in children.
**Option C:** This option is incorrect as it suggests an antiviral medication, which is not indicated for Mycoplasma pneumoniae infection.

**Clinical Pearl / High-Yield Fact**
In children, Mycoplasma pneumoniae infection often presents with atypical symptoms, such as headache, nausea, and vomiting, in addition to respiratory symptoms. The presence of "Morula" in the peripheral smear can be a diagnostic clue.

**Correct Answer: A. Azithromycin**