Achalasia is characterized by which of the following features?

Achalasia is a condition affecting the esophagus that is primarily characterized by the absence of peristalsis in the lower half of the esophagus. In patients with achalasia, the lower esophageal sphincter does not relax properly during swallowing, leading to a lack of coordinated muscle contractions that typically help move food down the esophagus. This absence of peristalsis results in a backup of food and can cause symptoms such as difficulty swallowing, regurgitation, and chest pain.

The other options relate to different gastrointestinal conditions or phenomena. Dilation of the stomach can occur in various cases, but it is not a feature of achalasia specifically; instead, it may result from prolonged food retention due to the esophageal blockage. Stenosis of the esophagus refers to a narrowing which can happen for various reasons, but achalasia is more about the functional absence of movement rather than a physical narrowing of the esophagus. Increased peristalsis is not characteristic of achalasia; rather, it is the reduced or absent peristaltic activity that defines the condition.