## **Core Concept**
The core concept being tested here is the calculation of the amount of drug that reaches the systemic circulation after oral administration, taking into account the bioavailability of the drug. Bioavailability is a pharmacokinetic parameter that represents the fraction of an administered dose of unchanged drug that reaches the systemic circulation.

## **Why the Correct Answer is Right**
To find the amount of drug reaching the systemic circulation, we multiply the dose administered by the bioavailability of the drug. Given that the dose of digoxin administered is 0.175 gm (or 175 mg) and the bioavailability is 70% (or 0.7), the calculation is as follows:
[ text{Amount reaching systemic circulation} = text{Dose} times text{Bioavailability} ]
[ text{Amount reaching systemic circulation} = 175 , text{mg} times 0.7 = 122.5 , text{mg} ]
So, the correct answer is the one that matches 122.5 mg.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not match the calculation of 122.5 mg.
- **Option B:** This option is incorrect for the same reason as Option A; it does not align with the calculated amount of drug reaching the systemic circulation.
- **Option D:** Similarly, this option does not match the correct calculation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that bioavailability is crucial for oral drugs because it directly affects the amount of drug that becomes available to produce its therapeutic effect. Drugs with low bioavailability may require higher doses to achieve the desired effect, but this also increases the risk of toxicity.

## **Correct Answer:** . 122.5 mg