**Question:** In type II Renal Tubular Acidosis (RTA), serum potassium level is -

A. High
B. Normal
C. Low
D. Unknown

**Correct Answer:** C. Low

**Core Concept:**
Type II RTA is a heterogeneous group of disorders characterized by impaired ammonia and acid excretion in the distal renal tubules. This results in accumulation of hydrogen ions (H+) and decreased bicarbonate reabsorption, leading to a failure in maintaining acid-base balance and electrolyte homeostasis.

**Why the Correct Answer is Right:**
In type II RTA, the primary defect involves the distal renal tubules where the primary cause is a deficiency of hydrogen ion (H+) secretion. This results in a relative increase in serum potassium (K+) levels due to the inability of the kidneys to excrete excess potassium efficiently. As a consequence, serum potassium levels are typically low in type II RTA.

**Why Each Wrong Option is Incorrect:**
A. High: This is incorrect as type II RTA results in a relative increase in serum potassium levels due to impaired H+ secretion.
B. Normal: This is incorrect as type II RTA results in a relative increase in serum potassium levels.
D. Unknown: This is incorrect because the correct answer is already provided as low serum potassium levels.

**Clinical Pearl:**
Type II RTA is a critical differential diagnosis to consider in patients presenting with hypokalemia (low serum potassium levels). Other causes of hypokalemia should also be ruled out, including medications (e.g., ACE inhibitors, diuretics), hypoaldosteronism (e.g., primary aldosteronopathy, Cushing's syndrome), and gastrointestinal losses (e.g., vomiting, diarrhea).

**Answer Explanation:**
In type II RTA, the primary defect involves the distal renal tubules leading to a deficiency of hydrogen ion (H+) secretion. This results in a relative increase in serum potassium levels. Hence, the correct answer is low serum potassium levels.