**Core Concept**
Stroke volume (SV) is the amount of blood ejected by the left ventricle of the heart in one contraction, measured in milliliters (mL). It is a critical determinant of cardiac output, which is the volume of blood pumped by the heart per minute.

**Why the Correct Answer is Right**
Stroke volume can be decreased due to several factors. **Option C: Increased afterload** can decrease stroke volume because afterload is the pressure against which the left ventricle must contract to eject blood into the aorta. When afterload increases (e.g., due to hypertension or aortic stenosis), the left ventricle must generate more force to eject the same amount of blood, which can decrease stroke volume. This is because the increased afterload reduces the left ventricle's ability to relax and fill with blood between contractions.

**Why Each Wrong Option is Incorrect**
**Option A:** Increased preload is incorrect because increasing preload (e.g., due to fluid overload) can actually increase stroke volume through the Frank-Starling mechanism, which states that the stroke volume of the heart increases in response to an increase in the volume of blood filling the heart (the end diastolic volume).

**Option B:** Decreased contractility is incorrect because while decreased contractility (e.g., due to heart failure) can decrease stroke volume, it is not the only factor that can decrease stroke volume, and other options are more specific and direct causes.

**Clinical Pearl / High-Yield Fact**
Remember the **Frank-Starling mechanism**: the heart's ability to increase stroke volume in response to increased preload (volume of blood in the ventricle at the end of diastole). This is a key concept in understanding how the heart adapts to changes in volume and pressure.

**Correct Answer: C. Increased afterload**