Final answer:
Esophageal obstruction is the most concerning potential adverse effect of Psyllium, which can occur if not taken with sufficient liquid, though constipation and diarrhea can also be side effects.
Step-by-step explanation:
The adverse effects of Psyllium, commonly known as Metamucil, largely relate to its fiber content and how it affects the digestive system. The most concerning adverse effect among the options provided is C) Esophageal obstruction. This condition can occur when the fiber supplement is not taken with enough liquid, leading to the supplement swelling and blocking the esophagus. However, other common problems with elimination like constipation and diarrhea can also occur if Psyllium is taken inappropriately or in incorrect dosages. While constipation can sometimes be an adverse effect, it is often what Psyllium is used to treat, making it an infrequent side effect when used properly. Keeping hydrated and following the correct dosage is essential when taking fiber supplements to mitigate such risks.