Which of the following can cause adult dysphagia?

Adult dysphagia, or difficulty swallowing, can be caused by a variety of medical conditions, with stroke being a significant and well-documented cause. When a stroke occurs, it can affect the brain regions responsible for controlling the muscles involved in swallowing. This disruption can lead to weakness or paralysis of the muscles used for swallowing, resulting in difficulty with safe and effective swallowing.

Normal aging generally leads to some changes in swallowing but does not typically cause dysphagia by itself. While some older individuals may experience swallowing difficulties due to age-related changes, it's often not as severe as those caused by a medical condition such as a stroke. Allergies could cause temporary throat irritation or discomfort but are not a direct cause of dysphagia. Therefore, among the listed options, stroke is clearly associated with the significant physiological changes that lead to adult dysphagia.