**Question:** The following is a passive process

A. Enzyme-catalyzed reaction
B. Autonomic nervous system reflex
C. Active transport
D. Immune response

**Core Concept:** Passive processes in medicine refer to events or phenomena that do not require active participation or intervention from an organism. They are typically influenced by external factors or occur spontaneously. In this context, passive processes do not involve significant energy expenditure or active cellular transport.

**Why the Correct Answer is Right:**

Correct Answer: **C. Active transport**

Active transport is an essential cellular process that requires energy (ATP) to transport substances across cell membranes against their concentration gradient. This process is highly relevant in maintaining cellular homeostasis and ensuring proper cellular function. In contrast, passive processes do not involve active cellular involvement or energy expenditure.

**Why Each Wrong Option is Incorrect:**

A. **Enzyme-catalyzed reaction:** Enzymes facilitate catalysis, speeding up chemical reactions without being consumed in the process. They do not represent a passive process.

B. **Autonomic nervous system reflex:** This refers to involuntary reflexes controlled by the autonomic nervous system. While reflexes are automatic responses, they do not fit the definition of a passive process as they involve neural pathways and are not driven solely by external factors.

D. **Immune response:** The immune response is an active process involving immune cells and mechanisms to defend the body against pathogens. It is not a passive process.

**Clinical Pearl:** Passive processes are often contrasted with active processes in medical exams. Understanding the differences between these processes can help distinguish between various physiological mechanisms and their implications in disease states or treatment plans.