**Core Concept**
Tobramycin is an aminoglycoside antibiotic that requires a loading dose to achieve therapeutic plasma concentrations quickly. The loading dose is calculated using the formula: Loading Dose (LD) = Desired Plasma Concentration (DPC) x Volume of Distribution (Vd).

**Why the Correct Answer is Right**
The desired plasma concentration (DPC) of tobramycin is 4 mg/L, and the volume of distribution (Vd) is 40 L. Using the formula, LD = DPC x Vd, we can calculate the loading dose: LD = 4 mg/L x 40 L = 160 mg. This is the amount of tobramycin that needs to be administered intravenously to achieve the desired plasma concentration rapidly.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the volume of distribution of tobramycin. Without knowing the Vd, we cannot accurately calculate the loading dose.

**Option B:** This option is incorrect because it uses the clearance (160 ml/min) instead of the volume of distribution (40 L) in the calculation. Clearance is not used to calculate the loading dose.

**Option C:** This option is incorrect because it does not provide a calculation for the loading dose. Without a calculation, we cannot determine the correct amount of tobramycin to administer.

**Option D:** This option is incorrect because it does not provide a calculation for the loading dose. Without a calculation, we cannot determine the correct amount of tobramycin to administer.

**Clinical Pearl / High-Yield Fact**
When administering a loading dose of an aminoglycoside like tobramycin, it's essential to ensure that the dose is not too high, as this can increase the risk of toxicity. The dose should be calculated carefully using the Vd and DPC to achieve therapeutic plasma concentrations without causing harm.

**Correct Answer:** C. 160 mg.