A 30 yr old female presents for routine checkup with asthma.she is on inhalational salbutamol, budesonide, and salmeterol. She complains of dyspnea.PEF was mildly improved. You have added zafirlukast to her prescription. What is the mechanism of action of zafirlukast?

**Core Concept**
Zafirlukast is a leukotriene receptor antagonist used in the management of asthma. It works by inhibiting the action of leukotrienes, which are pro-inflammatory mediators involved in the pathogenesis of asthma. Leukotrienes are produced through the lipoxygenase pathway and contribute to bronchoconstriction, inflammation, and mucus production in the airways.

**Why the Correct Answer is Right**
Zafirlukast specifically binds to the cysteinyl leukotriene receptor (CysLT1 receptor) on airway smooth muscle cells, platelets, and eosinophils. By blocking this receptor, zafirlukast inhibits the action of leukotrienes such as LTC4, LTD4, and LTE4, which are responsible for inducing bronchoconstriction, inflammation, and increased mucus production. This results in improved lung function and reduced symptoms in patients with asthma.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might refer to a mechanism involving the inhibition of histamine release, which is not the primary action of zafirlukast.
**Option B:** This option might suggest a mechanism involving the blockade of adrenergic receptors, which is not relevant to zafirlukast's action.
**Option C:** This option might imply a mechanism involving the inhibition of phosphodiesterase enzymes, which is not the correct mechanism of action for zafirlukast.

**Clinical Pearl / High-Yield Fact**
Zafirlukast is a useful addition to the treatment of asthma, particularly in patients who are not adequately controlled on inhaled corticosteroids and long-acting beta-agonists. It can be used as a monotherapy or in combination with other anti-asthmatic medications.

**Correct Answer: C. Inhibition of cysteinyl leukotriene receptor.**

✓ Correct Answer: B. Inhibition of actions of LTC4, D4