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6. The correct answer is C. Achalasia is an acquired esophageal motility disorder that is characterized by loss of enteric inhibitory neurons. The lower esophageal sphincter may exhibit increased tone in between swallows and fail to relax normally with a swallow. Peristalsis in the esophageal body is also abnormal. A swallow may not induce any peristalsis in the esophageal body or may produce simultaneous contractions along its entire length.

The esophageal body distal to the diaphragm (choice A) is relaxed in between swallows. The intraesophageal pressure at this point reflects the intra-abdominal pressure, which is slightly positive (5 mm Hg). During inspiration, the pressure inside the distal esophagus rises along with the intra-abdominal pressure; during expiration this pressure falls. The pressure in the esophageal body proximal to the diaphragm (choice B) reflects the intrathoracic pressure. It is slightly negative at the end of inspiration and slightly positive at the end of expiration.

Since the mouth and pharynx are open to the atmosphere, in between swallows, the pressure within the pharynx (choice D) is atmospheric (0 mm Hg). The pressure rises abruptly to a maximum of 100 mm Hg at the start of a swallow and returns to baseline within 0.5 seconds.

At rest, the pressure in the upper esophageal sphincter (choice E) can be as high as 60 mm Hg. It is maintained by the normal elasticity of the sphincteric structures, as well as by active contraction of the cricopharyngeal muscle, which composes most of the sphincter. Shortly after the pharyngeal muscles contract during a swallow, the upper esophageal sphincter relaxes as the tonic neural input to the cricopharyngeal muscle (skeletal muscle) is inhibited as part of the swallowing program. Function of this sphincter is unaffected by achalasia.

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