**Core Concept**
Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) that inhibits platelet aggregation by irreversibly inhibiting the enzyme cyclooxygenase (COX), which is necessary for the production of thromboxane A2 in platelets. This leads to a decrease in platelet aggregation and an increased risk of bleeding.

**Why the Correct Answer is Right**
Aspirin's mechanism of action is through the irreversible inhibition of COX-1, which is responsible for the production of thromboxane A2 in platelets. When aspirin is given to a patient already on heparin, the risk of bleeding increases because heparin also inhibits platelet aggregation by activating antithrombin III, which in turn inhibits thrombin. The combination of aspirin and heparin leads to a synergistic effect on platelet aggregation, significantly increasing the risk of bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because aspirin does not cause an increase in blood pressure, which is more related to the effects of NSAIDs on the renin-angiotensin system.
**Option B:** This option is incorrect because aspirin does not cause an increase in potassium levels, which is more related to the effects of certain diuretics on the renal tubules.
**Option C:** This option is incorrect because aspirin does not cause an increase in uric acid levels, which is more related to the effects of certain diuretics on urate reabsorption in the renal tubules.

**Clinical Pearl / High-Yield Fact**
When administering aspirin to patients, it is essential to consider the potential interactions with other anticoagulants, such as heparin, to minimize the risk of bleeding.

**Correct Answer:** D.