Patient description: The patient is a 16-year-old girl recently diagnosed with myasthenia gravis, class IIa. She complains of diplopia and weakness affecting in her upper extremities. She had a positive anti-AChR antibody test, and her single fiber electromyography (SFEMG) was positive. She is on acetylcholinesterase inhibitor treatment combined with immunosuppressants. But she still has some symptoms. She does not smoke or use illicit drugs. She is not sexually active, and her menses are regular. Her physical exam and lab studies are not remarkable for any other abnormalities. BP: 110/75 Hgb: 11 g/dl WBC: 8000 /mm3 Plt: 300000 /ml Creatinine: 0.5 mg/dl BUN: 10 mg/dl Beta hcg: negative for pregnancy

Clinical trial eligibility criteria: eligible ages (years): 2.0-999.0,  Kidney Disease Glomerular Disease Idiopathic Nephrotic Syndrome Focal Segmental Glomerulosclerosis Collapsing Glomerulopathy Children with newly diagnosed clinical idiopathic nephrotic syndrome between the ages of 2-18 years. 2. Children with previously diagnosed clinical idiopathic nephrotic syndrome between the age of 2-18 years at the time of diagnosis who sustain a relapse of nephrotic syndrome while off steroid therapy. A relapse is defined as a urine protein level of 2+ (100mg/dl) or greater on a first morning urine sample for 3 consecutive days. While a quantitative urine protein/creatinine ratio is more precise, this semi-quantitative definition accords with pediatric practice. Relapses will be treated according to standard clinical practice. 3. Children and adults who participate in NIH trials of remittive therapy for MCD/FSGS/CG (e.g. Podocyte dexamethasone or Retinoids for podocyte disease) will be eligible. 4. Also, we will study up to 10 healthy adult volunteers  IN ANY OF THE 1. Age less than 2 years. The rationale for excluding children under the age of 2 yr is that nephrotic syndrome is unusual in this age group and may have etiologies other than MCD and FSGS 2. Uncontrolled hypertension (a systolic or diastolic which is greater than 95% population mean for child s age and sex, while receiving therapy), azotemia (serum creatinine greater than 1.5mg/dL), gross hematuria at the time of presentation (clinical markers of primary or secondary GN). 3. Hypocomplementemia, positive ANA, anti-DS DNA, ASO, anti-DNase B, MPO, PR-3, Hepatitis B and C and HIV serologies. 2. In children or adults: 1. Clinical contra-indications to steroid therapy. 2. The presence of medical conditions that might interfere with the interpretation of results, such as cancer, diabetes, or a long history of hypertension. 3. Healthy adult volunteers: 1. age greater than 18 yrs; no history of hypertension, diabetes, or other chronic illness that might complicate interpretation of the results; not taking prescription medication. CKD STUDY: 1. Renal biopsy diagnosis of glomerular disease or interstitial nephritis or clinical diagnosis of diabetic nephropathy. 2. Mild or moderate renal insufficiency, defined as eGFR less than 60 ml/min/1.73m(2) but greater than 30 ml/min/1.73m(2) The presence of medical conditions that might interfere with the interpretation of results, such as cancer. STUDY: 1. Adults greater than 18 years old. Few children have diabetic nephropathy and the frequent blood draws would be problematic for children. 2. Type 1 or Type 2 diabetes. Later studies will be expanded to other glomerular diseases. 3. Urinary albumin/creatinine ratio greater than 30 mg/g. The rationale is to patients with macroproteinuria and patients with microalbuminuria. 4. Estimated GFR greater than 30 mL/min/1.73m(2). The rationale is to patients with severe CKD Allergy to ACE inhibitor or ARB 2. Current or planned pregnancy; urine HCG performed at start of study and on the day of iothalamate infusion. 3. Serum potassium greater than 5.0 mEq/L off angiotensin antagonist therapy 4. The presence of medical conditions which would make interpretation of results difficult such as cancer. 5. History of iodine allergy or severe asthma (history of emergency department visit or hospitalization). SEX IN 1. Age 18-30; Young adults are less likely to have undetected chronic renal disease. 2. BMI 18-27.5 kg/m(2); Obesity may alter renal function. 3. BP consistently less than 130/80; Pre-hypertension or hypertension may affect the kidney Any chronic illness that might interfere with interpretation of the study; certain illnesses that do not require therapy may be consistent with including mild asthma or psychiatric illness. 2. Cancer other than non-melanoma skin cancer in the past 5 years. 3. Need for any chronic or intermittent medication other than acetaminophen. 4. Women: amenorrhea, irregular menses, menstrual cycles less than 26 days or greater than 32 days, or use of contraceptive hormones (unwilling or unable to switch to non-hormonal means of contraception); Irregular cycle may be non-ovulatory and associated with abnormal sex hormone profiles, which will complicate data interpretation. 5. Use of nicotine or illicit drugs. 6. Febrile illness or urinary tract infection: postpone admission one week or more. SALT AND BLOOD IN 1. Men Age 18-35 2. BMI 18-25 kg/m(2) 3. BP consistently less than 130/80 Any chronic illness that might interfere with interpretation of the study; certain illnesses that do not require therapy may be consistent with including mild asthma or psychiatric illness 2. Cancer other than non-melanoma skin cancer in the past 5 years 3. Need for any chronic or intermittent medication other than acetaminophen 4. Use of nicotine or illicit drug 5. Febrile illness or urinary tract infection: postpone admission one week or more