This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: FMGE 2019, Subject name: Pharmacology
Which of the following drug causes postural hypotension commonly? 
Select and explain. 
A. ACE inhibitor, B. Beta blocker, C. Alpha blocker, D. Angiotensin receptor blockers.
C: Alpha blocker
Answer: C: Alpha blocker 
Explanation: Alpha 1A receptors are present on prostatic urethra whereas alpha 1B/1D are present on blood vessels. Drugs blocking both of these (like prazosin, terazosin, doxazosin and alfuzosin) are useful in BHP with hypeension but cause postural hypotension (First dose effect) as an adverse effect due to VENODILATION -hence causing pooling of blood in extremities -decreased venous return causing postaural hypotension. Selective alpha 1A blockers (tamsulosin and silodosin) treat symptoms of BHP without causing postural hypotension. Other drugs causing postural hypotension - ACE inhibitors- Enalapril Sodium nitroprusside.