**Question:** Which of the following drugs has maximum risk of dry mouth:

A. Cisapride
B. Metoclopramide
C. Donepezil
D. Quinidine

**Core Concept:** Dry mouth, also known as xerostomia, is a condition characterized by a decrease in saliva production, which can lead to various oral complications and discomfort. Salivary glands are primarily responsible for producing saliva, which plays a crucial role in maintaining oral health by lubricating the mouth, neutralizing acids, and facilitating digestion.

**Why the Correct Answer is Right:** Dry mouth is a known side effect of certain medications. In this case, quinidine is the correct answer because it is an antiarrhythmic drug that interferes with the salivary gland function. Quinidine binds to the Sjögren's syndrome A receptor (SSa), present on the salivary gland cells, thereby inhibiting the secretion of saliva. This disruption in salivary gland function leads to dry mouth as a side effect.

**Why Each Wrong Option is Incorrect:**

A. Cisapride: This drug is a prokinetic agent used to treat gastrointestinal motility disorders. Although it may cause dry mouth, the risk is generally lower compared to quinidine due to its limited effect on salivary gland function.

B. Metoclopramide: Similar to cisapride, metoclopramide is a prokinetic agent used to treat gastrointestinal disorders. Although it can cause dry mouth, the risk is generally lower than that of quinidine due to its minimal impact on salivary gland function.

C. Donepezil: Donepezil is an acetylcholinesterase inhibitor used in the treatment of Alzheimer's disease. While it can cause dry mouth, the risk is generally lower than that of quinidine due to its specific targeting of acetylcholinesterase and its effects on the central nervous system rather than salivary glands.

**Clinical Pearl:** It is essential for medical students and practitioners to be aware of the potential side effects of medications they prescribe and monitor patients for signs of dry mouth when prescribing these medications. Dry mouth can lead to oral infections, dental caries, and altered taste perception, affecting overall oral health and quality of life.