**Core Concept**
Methemoglobinemia is a condition characterized by an abnormal amount of methemoglobin in the blood, which can lead to a decrease in the oxygen-carrying capacity of hemoglobin. The reading of SpO2 (peripheral capillary oxygen saturation) can be affected by methemoglobinemia, as methemoglobin has a different optical absorption spectrum than normal hemoglobin.

**Why the Correct Answer is Right**
Methemoglobin has a reduced affinity for oxygen, and its presence in the blood can cause a decrease in the measured SpO2. This is because the pulse oximeter measures the absorption of light by hemoglobin and other chromophores in the blood, and methemoglobin can alter this absorption spectrum, leading to a falsely low reading of SpO2. In cases of methemoglobinemia, the SpO2 reading may be low even when the patient's actual oxygen saturation is normal.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because methemoglobinemia can indeed cause a decrease in the measured SpO2, as explained above.

**Option B:** This option is incorrect because while there are some conditions that can cause a discrepancy between SpO2 and arterial oxygen saturation (e.g., carbon monoxide poisoning), methemoglobinemia is a specific example of a condition that can cause this discrepancy.

**Option C:** This option is incorrect because methemoglobinemia can cause a decrease in the measured SpO2, but this is not necessarily a "normal" response.

**Option D:** This option is incorrect because while some conditions can cause a discrepancy between SpO2 and arterial oxygen saturation, methemoglobinemia is a specific example of a condition that can cause this discrepancy.

**Clinical Pearl / High-Yield Fact**
In cases of suspected methemoglobinemia, it's essential to consider the possibility of a discrepancy between SpO2 and arterial oxygen saturation, and to use other methods to assess oxygenation (e.g., arterial blood gas analysis).

**Correct Answer: D.