**Core Concept**
The patient's presentation is suggestive of a supraventricular tachycardia (SVT), a type of abnormal heart rhythm originating from the atria. SVT is characterized by a rapid heart rate, typically greater than 100 beats per minute, and can be associated with palpitations, shortness of breath, and dizziness.

**Why the Correct Answer is Right**
The initial therapy of choice for SVT is usually vagal maneuvers or adenosine, but since carotid massage and Valsalva maneuver were unsuccessful, the next step is to administer adenosine. Adenosine works by activating the A1 adenosine receptors on the AV node, which transiently blocks the AV node and interrupts the re-entrant circuit causing the SVT. This leads to a brief pause in the heart rate, allowing the patient to return to a normal sinus rhythm.

**Why Each Wrong Option is Incorrect**
**Option A:** Digoxin is not the initial therapy of choice for SVT as it can actually worsen the situation by increasing the conduction velocity through the AV node.
**Option B:** Beta-blockers may be used in the management of SVT, but they are not the initial therapy of choice, especially if the patient is hemodynamically unstable.
**Option C:** Calcium channel blockers, such as verapamil, can be used to treat SVT, but they are not the initial therapy of choice due to their potential to cause hypotension and bradycardia.

**Clinical Pearl / High-Yield Fact**
Adenosine is a potent vasodilator and can cause transient flushing, nausea, and headache in some patients. It's essential to have a pulse oximeter and an ECG monitor available when administering adenosine to quickly assess for any adverse effects.

**Correct Answer:** C. Adenosine