Which medication used for irritable bowel syndrome acts by a direct effect on smooth muscle?

• A. Block lymphocyte proliferation.
• B. Direct effect on smooth muscle.
• C. Monoclonal antibody that binds to tumor necrosis factor.
• D. 5-HT antagonist.

Answer: (B)

Some IBS meds relieve cramping by acting directly on GI smooth muscle. Antispasmodics like dicyclomine or hyoscyamine are muscarinic receptor antagonists in smooth muscle; by blocking acetylcholine’s effect, they reduce spontaneous contractions and spasm, easing pain and cramping. This direct action on muscle tissue distinguishes them from drugs that work by suppressing immune cell activity or by altering serotonin signaling. The other options involve blocking lymphocyte proliferation, targeting TNF with antibodies, or modulating motility through serotonin receptors, none of which rely on a direct smooth muscle effect.