**Core Concept**
Cystic fibrosis (CF) is a genetic disorder that affects the respiratory, digestive, and reproductive systems due to mutations in the CFTR gene, leading to abnormal chloride ion transport across epithelial membranes. This results in thick, sticky mucus production that can clog the airways and digestive tract. In infants, this abnormal mucus can cause intestinal obstruction.

**Why the Correct Answer is Right**
Meconium ileus is a condition where the meconium (the first stool of a newborn) is thick and sticky, causing an intestinal blockage in the ileum (the last part of the small intestine). This is a common complication in infants with CF due to the abnormal mucus production. The thick meconium fails to pass through the intestinal tract, leading to an obstruction. The CFTR gene mutation disrupts the sodium-chloride cotransporter in the intestinal epithelium, resulting in dehydrated, thick meconium.

**Why Each Wrong Option is Incorrect**
**Option B:** Loose motions are not typically associated with cystic fibrosis, as the condition is characterized by thick, sticky mucus production, not watery stools.
**Option C:** Vomiting can occur in infants with CF due to various reasons such as respiratory infections or gastroesophageal reflux, but it is not a direct consequence of the condition itself.
**Option D:** Constipation is not a typical presentation of cystic fibrosis in infants, as the condition is more often associated with meconium ileus or intestinal obstruction.

**Clinical Pearl / High-Yield Fact**
Cystic fibrosis can present with a wide range of symptoms, but meconium ileus is a classic early sign in infants. Remember the mnemonic "CF = Clogged Feces" to associate the condition with intestinal obstruction.

✓ Correct Answer: A. Meconium ileus