Except for purposes of sections 402(d), 402(e), 402(f), 423, and 425 of this title, the term “disability” means (A) inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which can be expected to result in death or has lasted or can be expected to last for a continuous period of not less than 12 months, or (B) blindness; and the term “blindness” means central visual acuity of 20/200 or less in the better eye with the use of a correcting lens. An eye which is accompanied by a limitation in the fields of vision such that the widest diameter of the visual field subtends an angle no greater than 20 degrees shall be considered for purposes of this paragraph as having a central visual acuity of 20/200 or less. The provisions of paragraphs (2)(A), (2)(B), (3), (4), (5), and (6) of section 423(d) of this title shall be applied for purposes of determining whether an individual is under a disability within the meaning of the first sentence of this paragraph in the same manner as they are applied for purposes of paragraph (1) of such section. Nothing in this subchapter shall be construed as authorizing the Commissioner of Social Security or any other officer or employee of the United States to interfere in any way with the practice of medicine or with relationships between practitioners of medicine and their patients, or to exercise any supervision or control over the administration or operation of any hospital.