Okay, let me tackle this question. The question is about the commonest side effect of short-acting beta-2 agonists used in asthma. The correct answer is likely to be something related to beta-2 receptor stimulation.

First, I need to recall what short-acting beta-2 agonists are. They include drugs like albuterol (salbutamol) and terbutaline. These are used for acute asthma attacks because they relax bronchial smooth muscles, leading to bronchodilation.

Now, the side effects. Beta-2 agonists can cause systemic effects because they might also stimulate beta-1 receptors in the heart, leading to tachycardia. But the most common side effect? When given via inhalation, systemic absorption is less, so local effects might be more prominent. However, when taken in high doses or with oral administration, systemic effects are more likely.

Wait, but the question specifies "commonest" side effect. Common ones could be tremor, palpitations, or hypokalemia. But which is the most common? I remember that tremor is a very frequent side effect, especially with inhaled beta-2 agonists. It's due to stimulation of beta-2 receptors in skeletal muscles. Palpitations (tachycardia) are also common because of beta-1 receptor stimulation in the heart. But which is more common?

Looking up some references, I see that tremor is listed as the most common side effect. For example, in clinical guidelines, tremor is often cited as the most frequent adverse effect of short-acting beta-2 agonists. So the answer is probably tremor.

Now, the options. Let's say the options are A) Tremor, B) Hypokalemia, C) Tachycardia, D) Hypoglycemia. Then the correct answer is A.

Why is hypokalemia not the answer? Because while beta-2 agonists can cause potassium to shift into cells (leading to hypokalemia), it's a less common side effect compared to tremor. Tachycardia is common but might not be the most common. Hypoglycemia is not typically associated with beta-2 agonists; in fact, they might increase blood glucose levels by promoting glycogenolysis.

Clinical pearl: Remember that tremor is the most common side effect of beta-2 agonists, especially with inhaled forms. Also, systemic effects are more pronounced with oral administration.

**Core Concept**
Short-acting beta-2 agonists (SABAs), such as albuterol, are bronchodilators used for acute asthma. Their side effects stem from beta-2 receptor stimulation in smooth muscles and unintended beta-1 receptor activation in the heart. Tremor is the most common adverse effect due to skeletal muscle beta-2 receptor overstimulation.

**Why the Correct Answer is Right**
Tremor (palmar hand tremor) occurs in ~40% of patients using SABAs. Beta-2 receptors in skeletal muscles mediate this effect when agonists bind, causing uncontrolled muscle contractions. Systemic absorption is minimal with inhaled forms, but high-dose nebulizers or frequent use can exacerbate this. Tremor is dose-dependent and resolves with drug clearance.

**Why Each Wrong Option is Incorrect**
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✓ Correct Answer: A. Muscle tremors