Which of the following symptoms is most characteristic of pyloric stenosis?

The presence of an olive-shaped mass felt in the abdomen is a hallmark sign of pyloric stenosis, especially in infants. This condition involves a narrowing of the pylorus, the opening from the stomach into the small intestine, which leads to gastric outlet obstruction. The hypertrophied pyloric muscle can be palpated as a distinct mass, often described as akin to an olive, in the right upper quadrant of the abdomen. This finding is significant in diagnosing pyloric stenosis and is usually accompanied by other symptoms such as projectile vomiting and dehydration.

Frequency of sneezing and runny nose, swollen lymph nodes, and bloody stools do not relate to pyloric stenosis. Sneezing and runny nose are more indicative of upper respiratory infections, while swollen lymph nodes can suggest various infections or systemic conditions. Bloody stools are often associated with gastrointestinal bleeding or other conditions affecting the intestines, rather than the obstructive process seen in pyloric stenosis. Thus, the olive-shaped mass is the most indicative clinical sign for this condition.