**Core Concept:**
The question refers to a newborn presenting with excessive frothing and choking during feeds, which could be due to a variety of conditions affecting the gastrointestinal (GI) tract or respiratory system. The APGAR score at birth is normal, which eliminates severe neonatal conditions as the primary cause.

**Why the Correct Answer is Right:**
The correct answer (D) is "Gastric aspiration x-ray". In this scenario, a plain X-ray of the chest is the most appropriate investigation to evaluate for aspiration pneumonitis or other GI tract issues causing the excessive frothing and choking. This examines the presence of gastric contents in the respiratory tract, which is the primary concern in this case.

**Why Each Wrong Option is Incorrect:**

A. "Blood gas analysis": This test is important for assessing respiratory acidosis or alkalosis but does not directly evaluate for gastric content aspiration.

B. "Biochemistry profile": This test is crucial for assessing the overall health status, organ functions, and electrolyte imbalances, but it does not address the specific concern of gastric content aspiration.

C. "Anti-emetic therapy": Administering anti-emetic medications would be inappropriate in this scenario as the focus is on identifying the cause (gastric aspiration) rather than treating its symptoms.

E. "Neonatal resuscitation": This is a resuscitation protocol for newborns and does not pertain to the investigation of excessive frothing and choking during feeds.

**Clinical Pearl:**
In neonates, gastric aspiration can occur due to immature reflexes, vomiting, or feeding difficulties. Gastric aspiration pneumonia is a serious condition, leading to respiratory distress and sepsis. Detecting aspiration with an appropriate investigation is crucial for timely management and preventing complications.

**Correct Answer Explanation:**
A plain chest X-ray helps to visualize the presence of gastric content in the respiratory tract, confirming the suspicion of gastric aspiration. This allows for an appropriate management plan, including referral to a specialist for further evaluation, gastrostomy tube placement if necessary, and respiratory support if required.

In conclusion, a plain chest X-ray is the appropriate investigation for an infant with excessive frothing and choking during feeds, as it helps diagnose gastric aspiration and guide appropriate management.