A patient complains of knee pain Routine investigations are unremarkable and still the patient is unsatisfied Urine turns black on standing what is the enzyme involved
**Core Concept**
The patient's complaint of knee pain and unremarkable investigations suggests a possible musculoskeletal issue. However, the key clue is the urine turning black on standing, which indicates the presence of hemoglobin or myoglobin in the urine, likely due to muscle breakdown.
**Why the Correct Answer is Right**
The enzyme involved in this scenario is **Lactate Dehydrogenase (LDH)**, specifically the isoform LDH-5. LDH-5 is predominantly found in cardiac and skeletal muscle cells. When these cells are damaged, LDH-5 is released into the bloodstream and eventually into the urine, where it can cause the urine to turn black on standing due to the oxidation of its substrate, pyruvate. This is a classic sign of rhabdomyolysis, a condition characterized by the breakdown of muscle tissue.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the scenario, as it does not involve muscle breakdown or the release of enzymes into the urine.
**Option B:** This option is incorrect because it is not related to the patient's symptoms or the laboratory findings.
**Option C:** This option is incorrect because, while it is involved in muscle metabolism, it is not the primary enzyme responsible for the urine turning black on standing.
**Option D:** This option is incorrect because it is not directly related to the patient's symptoms or the laboratory findings.
**Clinical Pearl / High-Yield Fact**
Rhabdomyolysis is a medical emergency that requires prompt diagnosis and treatment to prevent complications such as acute kidney injury.
**Correct Answer:** C. Lactate Dehydrogenase