**Core Concept**
Tall 'T' waves on an electrocardiogram (ECG) are indicative of hyperkalemia, a condition characterized by elevated potassium levels in the blood. This occurs when there is an imbalance between potassium intake and excretion, leading to a disruption in the normal functioning of cardiac cells.

**Why the Correct Answer is Right**
In hyperkalemia, the elevated potassium levels affect the cardiac action potential, causing the rapid depolarization phase (Phase 0) to become more rapid and prolonged. This results in tall, peaked 'T' waves on the ECG, which is a characteristic feature of hyperkalemia. The mechanism involves the blockage of sodium channels and the activation of potassium channels, leading to a decrease in the resting membrane potential and an increase in the excitability of cardiac cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because flat 'T' waves are associated with conditions such as ischemia, which affects the myocardial oxygen supply.
**Option B:** This choice is incorrect because left ventricular hypertrophy (LVH) is characterized by a deep 'S' wave and an inverted 'T' wave in lead V1.
**Option C:** This choice is incorrect because atrial fibrillation is a type of supraventricular tachycardia characterized by irregular, disorganized atrial activity, and not by tall 'T' waves.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that tall 'T' waves can be a sign of hyperkalemia, which can progress to more severe cardiac arrhythmias if not promptly treated.

**Correct Answer: D. Hyperkalemia**