This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
cGMP is second messenger for which hormone (s) 
Select and explain. 
A. Somatostatin, B. Atrial natruretic factor, C. Angiotensin II, D. Nitric Oxide (NO)
B: Atrial natruretic factor
Answer: B: Atrial natruretic factor 
Explanation: cGMP exist in two forms (Figure 2-29). One form has an extracellular amino-terminal domain that is a receptor, a single transmembrane domain, and a cytoplasmic poion with guanylyl cyclase catalytic activity. Three such guanylyl cyclases have been characterized. Two are receptors for atrial natriuretic peptide (ANP; also known as atrial natriuretic factor), and a third binds an Escherichia coli enterotoxin and the gastrointestinal polypeptide guanylin.(Ref: Ganong's review of medical physiology 23rd Edition page no.57)