To solve this question:

1. We have a required dosage of 42 mg for the patient.
2. The medication is available as 50 mg per 1 ml vial.

To find the required volume (in ml), we can use the proportion:

$\text{Volume (ml)} = \frac{\text{Required dosage (mg)}}{\text{Concentration (mg/ml)}}$

Substitute the values:

$\text{Volume (ml)} = \frac{42 \text{ mg}}{50 \text{ mg/ml}} = 0.84 \text{ ml}$

Thus, the correct answer is 0.84 ml.

Would you like a further breakdown or have additional questions?

Here are some related questions:

1. How would you adjust the calculation if the required dosage changed to 60 mg?
2. What if the concentration of the medication was 100 mg/ml instead?
3. How could you calculate the dosage in mg if you only know the volume administered?
4. Why is it important to ensure accuracy in dosage calculations for medications?
5. How would you calculate if the concentration was given in mg per liter instead?

Tip: Always double-check the concentration and required dosage units to avoid medication errors.