Okay, let's tackle this question. The user mentioned that a newborn hasn't passed meconium within 48 hours and has no external abnormalities. The correct answer is cystic fibrosis. I need to explain why.

First, the core concept here is the association between delayed meconium passage and certain conditions. I remember that cystic fibrosis is a common cause. The problem is with the viscosity of meconium due to thick secretions, which is part of the disease's pathology. The meconium can be sticky and hard to pass.

Now, why is cystic fibrosis the right answer? The disease is an autosomal recessive disorder caused by mutations in the CFTR gene. This affects the chloride channels, leading to thick mucus in various organs, including the intestines. The thick meconium can cause intestinal obstruction, even without external signs like anal atresia. So, if a baby doesn't pass meconium on time, especially with no obvious structural issues, CF should be considered.

The wrong options need to be addressed. Let's say the options include Hirschsprung's disease, which does cause intestinal obstruction but usually has other signs like abdominal distension. Down syndrome might have other features like facial characteristics. Spina bifida could have neurological signs. Each of these conditions has different presentations that would likely be present if they were the cause.

The clinical pearl here is to remember that delayed meconium passage without external anomalies is a red flag for cystic fibrosis. It's a high-yield fact for exams. Also, the newborn should be tested for CF with a sweat test or genetic testing in such cases.

**Core Concept**
Delayed passage of meconium beyond 48 hours of birth in a neonate with normal external anatomy raises suspicion for **cystic fibrosis (CF)**. This is due to thickened, viscous meconium caused by impaired chloride ion transport via defective **CFTR (cystic fibrosis transmembrane conductance regulator) protein**, leading to intestinal obstruction.

**Why the Correct Answer is Right**
Cystic fibrosis is an autosomal recessive disorder caused by mutations in the **CFTR gene**, which regulates chloride and sodium transport across epithelial surfaces. In the gastrointestinal tract, this leads to **meconium ileus**, a neonatal intestinal obstruction caused by abnormally thick and sticky meconium. Absence of external abnormalities (e.g., imperforate anus) differentiates CF from other structural causes of intestinal atresia. Diagnosis is confirmed via **sweat chloride testing** or genetic analysis.

**Why Each Wrong Option is Incorrect**
**Option A:** *Hirschsprung’s disease* causes congenital aganglionic megacolon but presents with abdominal distension, vomiting, and a "telescoped" rectum on exam, not isolated delayed meconium passage.
**Option B:** *Down syndrome* may have gastrointestinal malformations (e.g., duodenal atresia), but these typically present with bilious vomiting and characteristic facial features.
**Option C:** *Spina bifida* involves neurological deficits and visible spinal abnormalities, which are absent in this case.

**Clinical Pearl / High-Yield Fact**
**"Meconium ileus without external anomalies = think cystic fibrosis."** Remember that **meconium

✓ Correct Answer: C. Congenital aganglinosis