Final answer:
Infants with an abnormally thick pyloric muscle that blocks the passage into the duodenum often exhibit projectile vomiting and struggle to gain weight, as the condition prevents food from reaching the small intestine for digestion and nutrient absorption.
Step-by-step explanation:
In infants, when the pyloric muscle is abnormally thick and obstructs the opening into the duodenum, this condition is known as pyloric stenosis. One of the primary symptoms that may accompany this condition is projectile vomiting, as the stomach continues to contract attempting to move the chyme through the narrowed pyloric passage. This effort by the stomach often results in forceful vomiting, where the contents are expelled with great velocity. A consequence of this is poor weight gain or weight loss because the food is not making it into the duodenum for absorption. In contrast, the stomach is attempting to do its job by continuing to generate powerful muscular contractions to move the chyme through an obstruction it cannot overcome.