**Core Concept:** Posteroanterior (PA) and lateral chest radiographs are essential imaging modalities in diagnosing lung diseases. They provide valuable information about lung parenchyma, pleura, heart, mediastinum, and surrounding structures. The radiographs are commonly used in assessing pleural effusions, pulmonary parenchyma, and mediastinal masses.

**Why the Correct Answer is Right:** The correct answer is **D** (hypertrophic pyloric stenosis), as it is the most common cause of vomiting in infants, particularly in the first 3 months of life. In this condition, the pylorus (the narrow neck of the stomach that connects to the duodenum) becomes inflamed and thickened, leading to decreased gastric emptying and subsequent vomiting. The radiographic findings of this condition include diffuse enlargement of the stomach, a "double-bubble" appearance (due to the dilated stomach and proximal duodenum), and sometimes a "waterfall sign" (a clear line between the dilated stomach and duodenum).

**Why Each Wrong Option is Incorrect:**

A. **Cardiomegaly (A):** While cardiomegaly can be seen on chest radiographs, it is not a specific diagnosis for vomiting in infants. It may be associated with other conditions like congestive heart failure, valvular heart diseases, or pulmonary hypertension.

B. **Pneumonia (B):** Pneumonia is a bacterial, viral, or fungal infection causing inflammation and infiltrates in the lungs, which is not the correct diagnosis for infants with vomiting due to hypertrophic pyloric stenosis.

C. **Hypothyroidism (C):** Hypothyroidism is a hormonal disorder affecting the thyroid gland, which does not cause vomiting as the primary symptom in infants.

**Clinical Pearl:** A combination of clinical history, clinical examination, and radiographic findings is crucial in arriving at the correct diagnosis of hypertrophic pyloric stenosis. While the radiographic findings may be present, it is essential to consider the age group, onset, and duration of symptoms, as well as other associated symptoms like bilious vomiting, poor weight gain, and feeding difficulties. A high index of suspicion and clinical correlation are essential in making the diagnosis of this condition.