**Core Concept**
Tachycardia is a heart rate greater than 100 beats per minute, often caused by an imbalance between the sympathetic and parasympathetic nervous systems. Various factors can lead to tachycardia, including increased sympathetic tone, decreased parasympathetic tone, and underlying cardiac or systemic conditions.

**Why the Correct Answer is Right**
The correct answer is **Hydrocortisone**, a synthetic glucocorticoid that can cause tachycardia due to its systemic effects. Glucocorticoids can increase the heart rate by stimulating the release of catecholamines, such as adrenaline, from the adrenal glands. This leads to an increase in the sympathetic tone, causing the heart rate to rise.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Atenolol** is a beta-blocker that is actually used to treat tachycardia by blocking the effects of catecholamines on the heart. It decreases the heart rate, making it an incorrect option.
* **Option B:** **Digoxin** can cause bradycardia (slow heart rate) as a side effect, especially at high doses. It is not associated with tachycardia, making it an incorrect option.
* **Option C:** **Theophylline** is a bronchodilator that can cause tachycardia as a side effect due to its stimulatory effects on the heart. However, this is not the correct answer as it is indeed a cause of tachycardia.

**Clinical Pearl / High-Yield Fact**
Glucocorticoids, such as hydrocortisone, can cause a range of systemic effects, including tachycardia, hypertension, and hyperglycemia. It is essential to monitor patients on glucocorticoid therapy for these potential side effects.

**Correct Answer: D. Atenolol**