**Core Concept**
Renal tubular acidosis (RTA) refers to a group of disorders characterized by the kidneys' inability to regulate acid-base balance properly. This can occur due to various reasons, including impaired acid production, impaired bicarbonate resorption, or an inability to acidify urine. Each type of RTA has distinct pathophysiological mechanisms.

**Why the Correct Answer is Right**
Impaired acid production is not a characteristic of renal tubular acidosis. RTA is primarily associated with impaired bicarbonate resorption (Type 1 RTA) or an inability to acidify urine (Type 2 RTA). Impaired acid production is more closely related to conditions like pernicious anemia or autoimmune gastritis, where there is a deficiency of intrinsic factor leading to vitamin B12 deficiency. In these cases, the stomach fails to produce adequate hydrochloric acid, leading to impaired acid production.

**Why Each Wrong Option is Incorrect**
**Option B:** Impaired bicarbonate resorption is actually a hallmark of Type 1 RTA, where the kidneys fail to reabsorb bicarbonate ions properly, leading to a decrease in serum bicarbonate levels.

**Option C:** Inability to acidify urine is characteristic of Type 2 RTA (distal RTA), where the kidneys are unable to excrete hydrogen ions, resulting in alkaline urine.

**Option D:** Nephrolithiasis (kidney stones) is a common complication of RTA, particularly in Type 1 RTA, where the excessive excretion of calcium due to hypokalemia can lead to the formation of calcium stones.

**Clinical Pearl / High-Yield Fact**
A classic exam trap is to distinguish between Type 1 and Type 2 RTA. Remember that Type 1 RTA is characterized by impaired bicarbonate resorption, while Type 2 RTA is characterized by an inability to acidify urine.

**Correct Answer Line**
✓ Correct Answer: A. Impaired acid production.