This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Smooth Muscle, Subject name: Physiology
Smooth muscle contraction due to release of calcium, occurs because of ? 
Select and explain. 
A. Increased cAMP, B. Calcium combining with troponin C, C. Increased cGMP, D. Calmodulin-dependant myosin light chain kinase
D: Calmodulin-dependant myosin light chain kinase
Answer: D: Calmodulin-dependant myosin light chain kinase 
Explanation: In Smooth muscles, rise in intracellular leads to binding of 4 Ca++ ions with calmodulin. Calcium-calmodulin complex then activates myosin light chain kinase (MLCK). MLCK phosphorylates the regulatory light chain of myosin; this phosphorylation then leads to interaction of myosin with actin. Calmodulin is structurally and functionally related to troponin. Troponin is present in skeletal muscles and initiates contraction by causing a conformational change in the tropomyosin molecule which then uncovers the active sites on the actin filament.