Mercury toxicity manifests with a variety of clinical features primarily due to its effects on the nervous system, kidneys, and oral cavity.

- Tremors are one of the hallmark signs of chronic mercury poisoning, especially seen in occupational exposure. They are often termed "mercurial tremors" and affect fine motor skills.

- Gingivitis and other oral mucosal changes such as excessive salivation and stomatitis are common in mercury exposure, particularly with elemental mercury or mercurial compounds.

- Nephrotic syndrome can occur as a result of mercury-induced damage to the renal glomeruli, leading to proteinuria and edema. This is a recognized complication, especially with inorganic mercury exposure.

- Black water urine, however, is not associated with mercury toxicity. This clinical feature is typically seen in conditions causing massive intravascular hemolysis or rhabdomyolysis, such as malaria (blackwater fever) or severe hemolytic anemia, leading to dark-colored urine due to hemoglobin or myoglobin pigments. It is not a feature of mercury poisoning.

In summary, black water urine is not a clinical feature of mercury toxicity, whereas tremors, gingivitis, and nephrotic syndrome are well-documented manifestations.

Reference: Goldfrank's Toxicologic Emergencies, 11th Edition, Chapter 60: Heavy Metal Toxicity