Okay, let's tackle this question. The question is about a newborn who hasn't passed meconium in 48 hours. The options are missing, but the correct answer is supposed to be filled in. First, I need to recall the possible causes of delayed meconium passage. The first thing that comes to mind is Hirschsprung's disease, which is a congenital condition where a part of the large intestine lacks ganglion cells, leading to functional obstruction. This can prevent the passage of meconium.

The diagnostic procedure for Hirschsprung's is typically a rectal biopsy. There's also anorectal manometry, which measures the anal sphincter's function. Another test might be a barium enema, but that's less specific. The key here is to determine the correct diagnostic test.

So the core concept is Hirschsprung's disease and its diagnosis. The correct answer should be rectal biopsy. Let me check the other options. If the options include anorectal manometry, that's another possibility, but the gold standard is the biopsy. The other options might be things like abdominal X-ray, which isn't specific, or blood tests, which wouldn't help here.

Wait, but sometimes in the USMLE or similar exams, anorectal manometry is considered a first-line test before proceeding to biopsy. However, the definitive diagnosis is through biopsy. So the answer is probably rectal biopsy. The clinical pearl here is that delayed meconium passage in a newborn is a red flag for Hirschsprung's, and rectal biopsy is the gold standard. Let me structure the explanation accordingly.

**Core Concept**
Delayed meconium passage (>48 hours) in a newborn is a red flag for **Hirschsprung’s disease**, a congenital aganglionic megacolon. The diagnostic gold standard is **rectal biopsy** to confirm absence of ganglion cells in the affected bowel segment.

**Why the Correct Answer is Right**
**Rectal biopsy** (specifically, full-thickness or suction biopsy) is the definitive diagnostic test. It identifies the absence of **Meissner’s and Auerbach’s plexuses** in the submucosa and muscularis propria, respectively. This confirms the diagnosis of Hirschsprung’s disease, which occurs due to failed migration of neural crest cells during embryogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** *Anorectal manometry* is a **screening tool** to assess rectal reflexes but is not diagnostic. It may show absent rectoanal inhibitory reflex, but confirmation requires biopsy.
**Option B:** *Barium enema* may reveal a transition zone or microcolon but is **non-specific** and risks perforation in aganglionic segments.
**Option C:** *Abdominal X-ray* shows bowel gas patterns but cannot confirm Hirschsprung’s.
**Option D:** *Serum electrolyte testing* is irrelevant to diagnosing this condition.

**Clinical Pearl / High-Yield Fact**
Never forget that **delayed meconium passage** is a **classic presentation** of Hirschsprung’s. Rectal biopsy is the **gold standard**—do not confuse it with manometry (a preliminary test) or imaging alone.

**Correct Answer: C.

✓ Correct Answer: B. Contrast enema