**Core Concept**
The underlying principle being tested here is the clinical approach to a newborn with a suspected intestinal obstruction. This condition can be due to various causes such as congenital anomalies, Hirschsprung's disease, or meconium ileus. A timely and accurate diagnosis is crucial to prevent complications and ensure prompt treatment.

**Why the Correct Answer is Right**
The initial investigation of choice for a newborn with abdominal distention and vomiting, not passing meconium for 48 hours, is a Lower GI contrast study. This radiological examination helps to identify the level and cause of obstruction, which guides further management. The study involves the ingestion of a contrast agent that is visible on X-rays, allowing the radiologist to evaluate the bowel's patency and identify any potential obstructions.

**Why Each Wrong Option is Incorrect**
**Option A:** Manometry is a diagnostic tool used to measure the pressure and function of the gastrointestinal tract. While it can be useful in the evaluation of certain conditions like Hirschsprung's disease, it is not the initial investigation of choice for a newborn with suspected intestinal obstruction.

**Option B:** Genotyping for cystic fibrosis is not relevant in this scenario as meconium ileus is more commonly associated with cystic fibrosis. However, the initial investigation for suspected meconium ileus would be a Lower GI contrast study, not genotyping.

**Option D:** Serum trypsin immunoblot is used to diagnose cystic fibrosis in newborns, particularly in those with meconium ileus. However, this test is not the initial investigation of choice for a newborn with suspected intestinal obstruction.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that a newborn not passing meconium within 48 hours is a clinical sign that warrants further investigation for intestinal obstruction. The initial investigation should always aim to identify the level and cause of obstruction, which guides further management and treatment.

**✓ Correct Answer: C. Lower GI contrast study**