**Core Concept:**
Gross hematuria refers to the presence of a large number of red blood cells in the urine, making the urine appear visibly bloody. In the context of this question, the patient is a 65-year-old male smoker presenting with gross total painless hematuria. This indicates a concerning clinical picture that requires prompt investigation for diagnosis.

**Why the Correct Answer is Right:**
The correct answer, "Glomerulonephritis," is a group of diseases that affect the glomeruli within the kidneys. In this scenario, the patient presents with a smoking history, which is a known risk factor for glomerulonephritis. Additionally, the painless nature of the hematuria is also suggestive of a renal disorder. Glomerulonephritis can lead to hematuria due to inflammation and damage of the glomeruli, which filter blood and maintain fluid and electrolyte balance in the body.

**Why Each Wrong Option is Incorrect:**
A. **Lymphoma:** Although hematuria can be a symptom of lymphoma, the patient's age, smoking history, and painless presentation make lymphoma less likely.
B. **Renal calculi:** Renal calculi are often associated with pain and a history of urinary tract infections, which are not present in this case.
C. **Nephrolithiasis:** Similar to option B, nephrolithiasis is associated with pain and risk factors like obesity and dehydration, which are not relevant in this scenario.
D. **Atypical hemolytic uremic syndrome:** This is a rare disorder affecting platelets and red blood cells, typically involving children and young adults. The patient's age and smoking history make this diagnosis less likely.

**Clinical Pearl:**
In clinical practice, it is essential to consider the patient's age, risk factors, and clinical presentation while formulating a differential diagnosis. A systematic approach, including a detailed history and physical examination, can help narrow down the list of possible diagnoses and guide further investigations.

In this case, the patient's age, smoking history, and painless presentation make lymphoma, nephrolithiasis, and atypical hemolytic uremic syndrome less likely. Glomerulonephritis is a more probable diagnosis due to its association with smoking, painless hematuria, and the patient's age. Further evaluation with urinalysis, blood tests, and imaging studies can confirm the diagnosis and guide management.