**Core Concept**
The physiological important anion other than chloride refers to the anion that plays a crucial role in maintaining acid-base balance and electrolyte homeostasis in the body. This anion is responsible for the regulation of the bicarbonate buffering system and helps to maintain the pH of the blood within a narrow range.

**Why the Correct Answer is Right**
The correct answer is phosphate, as it is the second most abundant anion in the body after chloride. Phosphate plays a vital role in the bicarbonate buffering system, where it helps to regulate the pH of the blood by forming phosphate salts with hydrogen ions. This buffering system is essential for maintaining acid-base balance and preventing excessive fluctuations in blood pH. Additionally, phosphate is involved in the regulation of calcium metabolism and bone mineralization.

**Why Each Wrong Option is Incorrect**
**Option A:** Lactate is an important anion, but it is not a physiological anion in the same sense as chloride or phosphate. It is a byproduct of anaerobic metabolism and can accumulate in conditions such as lactic acidosis.

**Option B:** Bicarbonate is not an independent anion, but rather a conjugate base of carbonic acid. It is an important component of the bicarbonate buffering system, but it is not a free anion in the same way as phosphate or chloride.

**Option C:** This option is blank, please fill in the options A, B, C, and D.

**Clinical Pearl / High-Yield Fact**
The bicarbonate buffering system is a critical component of acid-base balance, and phosphate plays a vital role in regulating this system. A decrease in phosphate levels can lead to a decrease in bicarbonate levels, resulting in metabolic acidosis.

**Correct Answer:** C. Phosphate.