A patient admitted to the unit for dehydration tells the nurse that he has not had a bowel movement ...

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Bulk-forming laxatives can cause an intestinal obstruction or impaction if not enough water is consumed. The patient needs concurrent rehydration to prevent this.
Bright red stools would not be anticipated unless the patient has hemorrhoids, and there is no indication of this.
Black, tarry stools typically are seen if blood is present in the stool, no information suggests that the patient has gastrointestinal bleeding.
Pale, clay-colored stools may be apparent if the patient has a hepatic disorder, but no information suggests this.