## Core Concept
The question tests the understanding of the pharmacological effects of loop diuretics, such as furosemide, and their impact on electrolyte balance in the body. Furosemide works by inhibiting the sodium-potassium-chloride cotransporter in the thick ascending limb of the loop of Henle, leading to increased sodium and chloride excretion and, consequently, increased urine production. This action can also result in the loss of other electrolytes, including potassium and magnesium.

## Why the Correct Answer is Right
The patient's symptoms of fatigue and muscle weakness after starting furosemide are likely due to **hypokalemia** (low potassium levels) and possibly **hypomagnesemia** (low magnesium levels). Furosemide causes increased excretion of potassium ions in the urine, which can lead to hypokalemia if not adequately replaced. Hypokalemia is known to cause muscle weakness, fatigue, and other symptoms such as palpitations and muscle cramps.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, causes of muscle weakness would be considered in the context of electrolyte imbalances, drug side effects, or underlying disease progression.
- **Option B:** Similarly, without the content of Option B, we can infer that any choice not directly related to electrolyte imbalance or a known side effect of furosemide would be incorrect.
- **Option C:** Again, without specifics, any option not directly linking to the mechanism of action of furosemide or common complications of its use would be incorrect.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **potassium-sparing diuretics** or **potassium supplements** are often used in conjunction with loop diuretics like furosemide to prevent hypokalemia. Additionally, monitoring of serum electrolytes, including potassium and magnesium, is crucial in patients on long-term diuretic therapy to prevent such complications.

## Correct Answer: D. Hypokalemia.