**Core Concept:** Projectile vomiting in infants can be due to various causes, including organic conditions like malrotation, volvulus, and intussusception. In this scenario, the presenting symptoms and physical examination findings strongly suggest intestinal obstruction.

**Why the Correct Answer is Right:** Intestinal obstruction is a condition that prevents the normal movement of food through the gastrointestinal tract. In this case, the most likely cause is meconium ileus, which is a form of congenital obstruction affecting infants with cystic fibrosis (CF). Meconium ileus presents in neonates with symptoms like projectile vomiting, abdominal distension, and an "olive-like" mass on palpation. The visible peristaltic movements further support the diagnosis.

**Why Each Wrong Option is Incorrect:**

A. Volvulus: This involves twisting of the intestine, which would present with additional signs like pallor, hypotension, and shock.
B. Malrotation: This results in midgut volvulus, which typically causes symptoms like feeding intolerance, bilious vomiting, and abdominal distension.
C. Intussusception: It can present similarly to meconium ileus, but it is less common in neonates.
D. Intussusception: This condition is less common in neonates and would present with signs of obstruction and a palpable mass due to telescoping of the intestine.

**Clinical Pearl:** Meconium ileus is a rare but important differential diagnosis in neonates presenting with projectile vomiting, abdominal distension, and an "olive-like" mass on palpation. A high index of suspicion and appropriate investigations can lead to early diagnosis and timely intervention to prevent complications like bowel ischemia and necrosis.

**Correct Answer:** D. Intussusception

**Core Concept:** Intussusception is a condition where one part of the intestine telescopes into another part, typically due to a lead point. It is a less common cause of neonatal vomiting and abdominal distension, but it can present similarly to meconium ileus.

**Why the Correct Answer is Right:** Intussusception in neonates is often idiopathic and presents with symptoms like projectile vomiting, abdominal distension, and a palpable mass. In this case, the correct answer is D. Intussusception, as it is a plausible cause of the described symptoms and signs in this case.

**Why Each Wrong Option is Incorrect:**

A. Cystic fibrosis: This is not relevant to the specific symptoms described in the question.
B. Malrotation: While malrotation can cause intussusception, it is less likely in a neonate with projectile vomiting and abdominal distension.
C. Intussusception: While intussusception is a possibility, the other options are more likely causes of neonatal vomiting and abdominal distension.

**Clinical Pearl:** A high index of suspicion is crucial when evaluating neonates with vomiting and abdominal distension. Further investigations, such as abdominal ultrasound or X-ray, may be necessary to confirm the diagnosis and