**Core Concept**
Intolerance to a drug refers to the dose-dependent adverse effects that occur in a subset of individuals, even at doses that are typically considered safe. This phenomenon is distinct from hypersensitivity reactions, which are immune-mediated adverse effects. Intolerance is often related to the pharmacokinetic properties of the drug, such as its metabolism or absorption.

**Why the Correct Answer is Right**
The correct answer is related to the concept of the "therapeutic index" (TI), which is defined as the ratio of the dose that produces a therapeutic effect (ED50) to the dose that produces toxic effects (LD50). Intolerance occurs when the TI is narrow, meaning that the dose required to produce a therapeutic effect is close to the dose that produces adverse effects. In such cases, even a small increase in dose can lead to intolerable side effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not address the concept of dose-dependent adverse effects. Intolerance is not simply a matter of individual variability in drug response.

**Option B:** This option is incorrect because it implies that intolerance is an all-or-nothing phenomenon, whereas in reality, it is a dose-dependent effect.

**Option C:** This option is incorrect because it suggests that intolerance is related to the frequency of administration, rather than the dose itself.

**Option D:** This option is incorrect because it implies that intolerance is a fixed property of the drug, rather than a property of the individual.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient for intolerance to a medication, it's essential to consider the therapeutic index and the dose-response relationship for that particular drug. A narrow TI may indicate that the patient is at risk for intolerable side effects, even at standard doses.

**Correct Answer:** C.