Regarding alpha-1 adrenergic blockers, all of the following statements are correct EXCEPT:

Question

Accepted Answer

C. Phenoxybenzamine is a reversible, competitive antagonist used acutely in pheochromocytoma management. ## Understanding Alpha-1 Blocker Pharmacology

### Mechanism & Classification

**Key Point:** Alpha-1 blockers are classified as either **non-selective** (prazosin, doxazosin, terazosin) or **selective** (tamsulosin, alfuzosin, silodosin) based on their receptor subtype specificity.

### Analysis of Each Statement

| Statement | Correctness | Explanation |
|-----------|-------------|-------------|
| Prazosin first-dose syncope | ✓ Correct | Prazosin causes sudden, profound vasodilation → orthostatic hypotension → syncope, especially with first dose. Mitigated by low initial dosing and bedtime administration. |
| Doxazosin vs terazosin half-life | ✓ Correct | Doxazosin t½ = 22 hrs; terazosin t½ = 12 hrs. Doxazosin allows once-daily dosing; terazosin requires twice-daily dosing. |
| Tamsulosin selectivity | ✓ Correct | Tamsulosin is **10-fold selective** for alpha-1A (prostate/bladder neck) over alpha-1B (vascular smooth muscle). Minimal vasodilation and hypotension compared to non-selective blockers. |
| Phenoxybenzamine reversibility | ✗ **INCORRECT** | Phenoxybenzamine is an **irreversible, non-competitive antagonist** — it forms a covalent bond with alpha receptors. It is the drug of choice for chronic pheochromocytoma prep (not acute). Phentolamine is the reversible agent used acutely. |

### Clinical Pearl:

**High-Yield:** Phenoxybenzamine's irreversibility means its effects last 3–4 days even after the drug is stopped — this is why it is preferred for preoperative pheochromocytoma blockade (sustained protection). Phentolamine, being reversible, is used only for acute hypertensive crises during anesthesia or tumor manipulation.

### Mnemonic:

**PHAT-PH:** 
- **P**razosin → **P**rostate & **H**ypotension (first-dose syncope)  
- **H**alf-life: **D**oxazosin > **T**erazosin  
- **A**lpha-1A selective → **T**amsulosin (minimal vascular effects)  
- **P**henoxybenzamine → **Irreversible** (covalent bond)  
- **P**hentolamine → **Reversible** (acute use)

[cite:KD Tripathi 8e Ch 10]

Answer

A. Prazosin causes first-dose syncope due to sudden vasodilation and orthostatic hypotension

Answer

B. Doxazosin has a longer half-life than terazosin and requires once-daily dosing

Answer

D. Tamsulosin is selective for alpha-1A receptors in the prostate and causes minimal vasodilation

Regarding alpha-1 adrenergic blockers, all of the following statements are correct EXCEPT:

Explanation

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