**Core Concept**
The FIGLU excretion test is a diagnostic tool used to assess the functional capacity of the pyrimidine biosynthesis pathway, specifically the enzyme dihydroorotate dehydrogenase (DHODH) in the liver. This pathway is crucial for the de novo synthesis of pyrimidines from non-pyrimidine precursors.

**Why the Correct Answer is Right**
The FIGLU (formiminoglutamate) excretion test relies on the fact that when the DHODH enzyme is deficient, the body is unable to convert formiminoglutamate (FIGLU) to formiminoglycine (FIG). As a result, FIGLU accumulates and is excreted in the urine. This test is particularly useful in diagnosing dihydroorotate dehydrogenase deficiency, a rare genetic disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the FIGLU excretion test is not used to estimate deficiency of the enzyme carbamoyl phosphate synthetase I (CPS I), which is involved in the urea cycle.
**Option B:** This option is incorrect because the FIGLU excretion test is not used to estimate deficiency of the enzyme ornithine transcarbamylase (OTC), which is also involved in the urea cycle.
**Option C:** This option is incorrect because the FIGLU excretion test is not used to estimate deficiency of the enzyme argininosuccinate synthase (ASS), which is involved in the urea cycle.

**Clinical Pearl / High-Yield Fact**
The FIGLU excretion test is a useful diagnostic tool for dihydroorotate dehydrogenase deficiency, a rare genetic disorder that can lead to severe metabolic disturbances if left untreated.

**Correct Answer: D.**